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