do the right thing when TrackingText::offset is changed

libs/canvas/canvas/item.h

@@ -86,6 +86,10 @@ public:
 	 */
 	virtual void add_items_at_point (Duple /*point*/, std::vector<Item const *>& items) const;
 
+        /** Return true if the item covers @param point, false otherwise.
+         * 
+         * The point is in window coordinates 
+         */
         virtual bool covers (Duple const &) const;
 
 	/** Update _bounding_box and _bounding_box_dirty */

libs/canvas/tracking_text.cc

@@ -128,17 +128,23 @@ TrackingText::show_and_track (bool tx, bool ty)
 void
 TrackingText::set_x_offset (double o)
 {
+        begin_change ();
 	offset.x = o;
+        end_change ();
 }
 
 void
 TrackingText::set_y_offset (double o)
 {
+        begin_change ();
 	offset.y = o;
+        end_change ();
 }
 
 void
 TrackingText::set_offset (Duple const & d)
 {
+        begin_change ();
 	offset = d;
+        end_change ();
 }