Add editable resize/rotate/skew handles for shapes and images

Shapes and images can now be resized, rotated, and skewed directly
on the canvas, not just moved. Both share one small transform
struct (rotation, then skew, then scale, anchored on a movable
pivot) and one handle widget, so a corner drag, an edge skew, or
grabbing the rotate handle behaves the same way and runs through
the same matrix math everywhere, instead of every item type
reinventing its own.

Shapes also gained a proper pen tool (bezier paths, corner/smooth/
symmetric nodes), arc support, and mirroring. Images gained
non-destructive cropping and colour-keyed transparency, both
remember their own settings, so reopening the dialog picks up
where you left off instead of starting over.

Properties dialogs for both were extended to match (position,
size, angle, skew), with undo/redo wired through for every handle
drag.

Old XML files can read easily as the transformation is only added
if needed and the old syntax is still used and understood if it is
not needed.

sources/ui/imagepropertieswidget.h

@@ -48,14 +48,33 @@ class ImagePropertiesWidget : public PropertiesEditorWidget
 		void updateUi() override;
 
 	private slots:
-		void on_m_scale_slider_valueChanged(int value);
+		void on_m_width_sb_valueChanged(double value);
+		void on_m_height_sb_valueChanged(double value);
+		void on_m_angle_sb_valueChanged(double value);
+		void on_m_skew_x_sb_valueChanged(double value);
+		void on_m_skew_y_sb_valueChanged(double value);
+		void on_m_lock_ratio_tb_toggled(bool checked);
+		void on_m_restore_ratio_pb_clicked();
 		void on_m_lock_pos_cb_clicked();
 
 	private:
 		Ui::ImagePropertiesWidget *ui;
 		DiagramImageItem *m_image;
 		bool m_movable;
-		qreal m_scale;
+		// All tracked independently -- the image may already be
+		// non-uniformly scaled, rotated, and/or skewed via the resize/
+		// rotate/skew handles before this dialog is even opened, and
+		// undo/reset have to restore each to whatever it actually was.
+		qreal m_scaleX;
+		qreal m_scaleY;
+		qreal m_rotation;
+		qreal m_skewX;
+		qreal m_skewY;
+		// Guards the width/height spinboxes' mutual updates when
+		// "Conserver les proportions" is checked, so setting one
+		// programmatically in response to the other doesn't re-trigger
+		// its own valueChanged and recurse.
+		bool m_updating_ratio = false;
 };
 
 #endif // IMAGEPROPERTIESWIDGET_H