Fix unwanted moving part in element editor.

Fix an unwanted behavior when the properties dock widget is displayed :

1 there is no selection
2 the dock widget width is set to minimum
3 select a part, the dock widget gain new widgets used to edit the
current selected part and the width of the dock grow so the width of the
QGraphicsView is reduced and cause a mouse move event.
When this case occur the part is moved but they should not.

sources/editor/graphicspart/customelementgraphicpart.h

@@ -328,6 +328,7 @@ class CustomElementGraphicPart : public QGraphicsObject, public CustomElementPar
 		Color _color;
 		bool _antialiased;
 		QPointF m_origin_pos;
+		bool m_first_move;
 };
 
 typedef CustomElementGraphicPart CEGP;