add Item::redraw() convenience method

libs/canvas/canvas/item.h

@@ -56,6 +56,8 @@ public:
 	Item (Group *, Duple);
 	virtual ~Item ();
 
+        void redraw () const;
+
 	/** Render this item to a Cairo context.
 	 *  @param area Area to draw, in **window** coordinates
 	 *

libs/canvas/item.cc

@@ -328,6 +328,14 @@ Item::width () const
 	return 0;
 }
 
+void
+Item::redraw () const
+{
+	if (_bounding_box && _canvas) {
+		_canvas->request_redraw (item_to_canvas (_bounding_box.get()));
+	}
+}	
+
 void
 Item::begin_change ()
 {