Add "Export to SVG" to the element editor

Implements discussion #605. The diagram editor can already export a folio
to SVG; the element editor, where a single .elmt symbol is drawn, had no
export capability at all -- confirmed by grepping its header for "export"
before starting: nothing.

## Renders the live scene, not ElementPictureFactory's cache

The discussion proposed sourcing this from ElementPictureFactory's cached
per-element QPicture (m_pictures_H), the one used for the elements-panel
preview icons. Checked that cache's actual invalidation before building on
it: nothing in the editor ever tells it to drop an entry on edit, and it is
keyed by the element's on-disk uuid. So for any element already previewed
once in the panel, exporting from the cache would silently produce stale
content after any edit; for a brand-new, never-saved element, no entry
would exist at all. Neither is acceptable for a File > Export action a
user expects to reflect what's on screen right now.

Renders ElementScene directly instead, the same way
ExportDialog::generateSvg() already renders the live Diagram for the
diagram editor's own SVG export: no new drawing logic, only a new playback
target (QSvgGenerator instead of the screen), sized to the element's own
content bounds via the existing elementSceneGeometricRect() helper.

## Hotspot cross excluded from the export

ElementScene::drawForeground() draws the red origin/hotspot cross on every
render() call, unconditionally -- it's an editing aid, not part of the
element being drawn, and diagram editor's SVG export has no equivalent
problem since Diagram doesn't draw one. Added a settable
hotspotVisible flag, defaulting to true (the existing editing view is
completely unaffected) and turned off only for the duration of the export
render() call.

## Verified end-to-end via a real Xvfb session, not just a build

Opened a real shipped element (en_60617_05_06_04.elmt, "Phototransistor"),
exported it, and rendered the resulting SVG back to a bitmap with a small
QSvgRenderer-based harness -- pixel-identical in shape to the element as
shown in the editor. Confirmed the file is valid XML and contains no
red/#ff0000 stroke (the hotspot cross did not leak in).

Then the case the whole "render live, not cached" decision was about:
opened the same element, drew a new line with the line tool, and exported
again *without saving*. The new line is present in the exported SVG.
git status on the source .elmt file after both exports shows it completely
untouched -- the export is read-only and reflects live, unsaved editor
state, exactly the property a cache-based implementation would have gotten
wrong.

Built clean, no new warnings.

sources/editor/elementscene.cpp

@@ -317,6 +317,10 @@ void ElementScene::contextMenuEvent(QGraphicsSceneContextMenuEvent *event)
 */
 void ElementScene::drawForeground(QPainter *p, const QRectF &)
 {
+	if (!m_hotspot_visible) {
+		return;
+	}
+
 	p -> save();
 
 	// desactive tout antialiasing, sauf pour le texte

sources/editor/elementscene.h

@@ -91,6 +91,8 @@ class ElementScene : public QGraphicsScene
 
 		QPointer<CustomElementGraphicPart> m_single_selected_item;
 
+		bool m_hotspot_visible = true;
+	
 		// methods
 	public:
 		ElementData elementData();
@@ -133,6 +135,14 @@ class ElementScene : public QGraphicsScene
 		void addItems(QVector<QGraphicsItem *> items);
 		void removeItems(QVector<QGraphicsItem *> items);
 
+			/// Whether drawForeground() draws the red hotspot cross. On by
+			/// default (the normal editing view); turned off around a
+			/// render() call meant to capture the element's actual content
+			/// only, e.g. exporting to SVG -- the cross is an editing aid,
+			/// not part of the drawn symbol.
+		void setHotspotVisible(bool visible) {m_hotspot_visible = visible;}
+		bool hotspotVisible() const {return m_hotspot_visible;}
+	
 	protected:
 		void mouseMoveEvent         (QGraphicsSceneMouseEvent *) override;
 		void mousePressEvent       (QGraphicsSceneMouseEvent *) override;

sources/editor/ui/qetelementeditor.cpp

@@ -53,6 +53,8 @@
 
 #include <QSettings>
 #include <QActionGroup>
+#include <QFileDialog>
+#include <QSvgGenerator>
 
 /**
  * @brief QETElementEditor::QETElementEditor
@@ -1377,6 +1379,84 @@ bool QETElementEditor::on_m_save_as_file_action_triggered()
 	return false;
 }
 
+/**
+	@brief QETElementEditor::on_m_export_svg_action_triggered
+	Export the element currently open in this editor to a standalone SVG
+	file.
+
+	Renders the live ElementScene directly, the same way
+	ExportDialog::generateSvg() renders the live Diagram for the diagram
+	editor's own SVG export -- not ElementPictureFactory's cached picture.
+	That cache is keyed by the element's saved-to-disk uuid and is never
+	invalidated on edit (nothing in this editor ever tells it to), so it
+	would silently export stale content for any element already previewed
+	once in the elements panel, and nothing at all for one that has never
+	been saved. Rendering the scene directly has neither problem and
+	always reflects exactly what is currently on screen, saved or not.
+	@return true if the file was written
+*/
+bool QETElementEditor::on_m_export_svg_action_triggered()
+{
+	QString fn = QFileDialog::getSaveFileName(
+			this,
+			tr("Exporter en SVG", "dialog title"),
+			m_file_name.isEmpty() ? QETApp::customElementsDir() : QDir(m_file_name).absolutePath(),
+			tr("Image SVG (*.svg)", "filetypes allowed when exporting an element to SVG"));
+
+	if (fn.isEmpty()) {
+		return false;
+	}
+	if (!fn.endsWith(".svg", Qt::CaseInsensitive)) {
+		fn += ".svg";
+	}
+
+	QFile file(fn);
+	if (!file.open(QIODevice::WriteOnly)) {
+		QMessageBox::critical(this, tr("Échec de l'export"),
+				      tr("Impossible d'écrire dans le fichier « %1 ».").arg(fn));
+		return false;
+	}
+
+		//Margin-less bounding rect of the element's own drawn content
+		//(lines, rects, terminals, text...), excluding the origin cross
+		//and other editor-only decoration -- see
+		//ElementScene::elementSceneGeometricRect()'s own doc comment.
+		//Falls back to itemsBoundingRect() for the rare element made up
+		//only of item types that helper deliberately excludes.
+	QRectF source_rect = m_elmt_scene->elementSceneGeometricRect();
+	if (source_rect.isEmpty()) {
+		source_rect = m_elmt_scene->itemsBoundingRect();
+	}
+	constexpr qreal margin = 5.0;
+	source_rect.adjust(-margin, -margin, margin, margin);
+
+	QSize target_size = source_rect.size().toSize();
+	if (target_size.isEmpty()) {
+		target_size = QSize(1, 1);
+	}
+
+	QSvgGenerator svg_engine;
+	svg_engine.setSize(target_size);
+	svg_engine.setViewBox(QRect(QPoint(0, 0), target_size));
+	svg_engine.setOutputDevice(&file);
+
+	QPainter svg_painter(&svg_engine);
+	svg_painter.setRenderHint(QPainter::Antialiasing, true);
+	svg_painter.setRenderHint(QPainter::TextAntialiasing, true);
+
+		//The hotspot cross is ElementScene::drawForeground()'s editing aid,
+		//drawn unconditionally on every render() call including this one
+		//unless told not to -- it is not part of the element being
+		//exported.
+	m_elmt_scene->setHotspotVisible(false);
+	m_elmt_scene->render(&svg_painter, QRectF(QPointF(0, 0), target_size), source_rect);
+	m_elmt_scene->setHotspotVisible(true);
+
+	svg_painter.end();
+
+	return true;
+}
+
 void QETElementEditor::on_m_reload_action_triggered()
 {
 	//If user already edit the element, ask confirmation to reload

sources/editor/ui/qetelementeditor.h

@@ -87,6 +87,7 @@ class QETElementEditor : public QMainWindow
 		void on_m_open_action_triggered();
 		void on_m_open_from_file_action_triggered();
 		bool on_m_save_as_file_action_triggered();
+		bool on_m_export_svg_action_triggered();
 		void on_m_reload_action_triggered();
 		void on_m_quit_action_triggered();
 		void on_m_deselect_all_action_triggered();

sources/editor/ui/qetelementeditor.ui

@@ -40,6 +40,7 @@
     <addaction name="m_save_action"/>
     <addaction name="m_save_as_action"/>
     <addaction name="m_save_as_file_action"/>
+    <addaction name="m_export_svg_action"/>
     <addaction name="separator"/>
     <addaction name="m_reload_action"/>
     <addaction name="separator"/>
@@ -262,6 +263,15 @@
     <string>Enregistrer dans un fichier</string>
    </property>
   </action>
+  <action name="m_export_svg_action">
+   <property name="icon">
+    <iconset resource="../../../qelectrotech.qrc">
+     <normaloff>:/ico/22x22/document-export.png</normaloff>:/ico/22x22/document-export.png</iconset>
+   </property>
+   <property name="text">
+    <string>Exporter en SVG</string>
+   </property>
+  </action>
   <action name="m_reload_action">
    <property name="icon">
     <iconset resource="../../../qelectrotech.qrc">