Add toolbar widget for edit size of handler in diagram editor.

Add a combo box in the tool bar of diagram editor
to quickly change the size of the graphics handler item.

The sarto commit :D

NOTE
only available for diagram editor, element editor will
come later.

sources/QetGraphicsItemModeler/qetgraphicshandleritem.h

@@ -37,6 +37,7 @@ class QetGraphicsHandlerItem : public QGraphicsItem
 {
 	public:
 		QetGraphicsHandlerItem(qreal size = 10);
+		void setSize(qreal size);
 		QRectF boundingRect() const override;
 		
 		enum { Type = UserType + 1200};