Add "rotate group" to actually rotate a selection as a whole

RotateSelectionCommand's existing "Pivoter" action (Space) only ever
bumps each selected item's own rotation property -- QGraphicsItem's
setRotation() spins an item around its own local origin and never
touches pos(). Select three elements arranged in a row and rotate:
each spins 90 degrees individually, but the row stays a row. That's
"rotate each item," not "rotate the group."

Add a rotate_as_group parameter to RotateSelectionCommand (default
false, so the existing action and its one call site are unchanged).
When set, it computes a shared pivot once -- the bounding-box center
of the whole selection -- and queues a second, parallel "pos"
QPropertyUndoCommand alongside the existing "rotation" one, rotating
each item's position around that pivot by the same angle.

Scoped the position change to Element/IndependentTextItem/
DiagramImageItem only: these are the only selectable types with
scene-space pos(). ConductorTextItem, DynamicElementTextItem and
ElementTextItemGroup are all parent-relative children (confirmed by
reading their constructors), so when their owning Element is also
selected and gets its own pos() rotated, they're carried along for
free by Qt's normal parent/child transform propagation -- exactly
what the existing "skip rotation if parent is also selected" guard
already assumes for those three cases.

Exposed as a new, separate action ("Pivoter le groupe", Shift+Space)
next to the existing one rather than changing Space's behavior, since
some workflows may rely on the current per-item rotation.

sources/undocommand/rotateselectioncommand.h

@@ -21,10 +21,12 @@
 #include <QUndoCommand>
 #include <QPointer>
 #include <QHash>
+#include <QPointF>
 
 class Diagram;
 class ConductorTextItem;
 class QPropertyUndoCommand;
+class QGraphicsItem;
 
 /**
 	@brief The RotateSelectionCommand class
@@ -33,13 +35,15 @@ class QPropertyUndoCommand;
 class RotateSelectionCommand : public QUndoCommand
 {
 	public:
-		RotateSelectionCommand(Diagram *diagram, qreal angle=90, QUndoCommand *parent=nullptr);
+		RotateSelectionCommand(Diagram *diagram, qreal angle=90, QUndoCommand *parent=nullptr, bool rotate_as_group=false);
 		void undo() override;
 		void redo() override;
 
 		bool isValid();
 
 	private:
+		void addGroupPositionUndo(QGraphicsItem *item, const QPointF &pivot, qreal angle);
+
 		Diagram *m_diagram =nullptr;
 
 		QList<QPointer<ConductorTextItem>> m_cond_text;