Add position-lock checkbox to element properties (issue #801)

Elements already inherited QetGraphicsItem::isMovable()/setMovable() --
the same mechanism images and drawn shapes use for their "lock position"
checkbox -- but nothing exposed it in the element properties panel, and
Element::toXml()/fromXml() never persisted it.

- ElementPropertiesWidget::generalWidget(): add a "Verrouiller la
  position" checkbox mirroring ShapeGraphicsItemPropertiesWidget's
  m_lock_pos_cb, toggling the element's inherited setMovable().
- Element::toXml()/fromXml(): persist is_movable, same attribute name
  and default-true behavior as DiagramImageItem/QetShapeItem.

Verified via headless --resave round-trip: is_movable="0" survives
load -> save unchanged, existing elements without the attribute default
to movable.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HdWpDp3TrPKbHnv7YUcNJj

sources/ui/elementpropertieswidget.cpp

@@ -30,6 +30,7 @@
 #include "masterpropertieswidget.h"
 #include "plclinkwidget.h"
 
+#include <QCheckBox>
 #include <QLabel>
 #include <QUndoStack>
 #include <QVBoxLayout>
@@ -396,6 +397,18 @@ QWidget *ElementPropertiesWidget::generalWidget()
 	label->setTextInteractionFlags(Qt::TextEditorInteraction);
 	vlayout_->addWidget(label);
 
+		//checkbox to lock the element position on the diagram
+		//(same mechanism already used by images and drawn shapes)
+	QCheckBox *lock_pos_cb = new QCheckBox(tr("Verrouiller la position"), general_widget);
+	lock_pos_cb->setChecked(!m_element->isMovable());
+	QPointer<Element> element = m_element;
+	connect(lock_pos_cb, &QCheckBox::clicked, this, [element](bool checked) {
+		if (element) {
+			element->setMovable(!checked);
+		}
+	});
+	vlayout_->addWidget(lock_pos_cb);
+
 		//widget for the pixmap
 	QLabel *pix = new QLabel(general_widget);
 	vlayout_->addWidget(pix, 0, Qt::AlignHCenter);