fix bad lifetime mgmt for VerboseCursor class

gtk2_ardour/editing_context.cc

@@ -253,6 +253,7 @@ EditingContext::~EditingContext()
 	ActionManager::drop_action_group (velocity_actions);
 	ActionManager::drop_action_group (zoom_actions);
 
+	delete _verbose_cursor;
 	delete grid_lines;
 }
 

gtk2_ardour/verbose_cursor.h

@@ -28,7 +28,7 @@ namespace ArdourCanvas {
 	class TrackingText;
 }
 
-class VerboseCursor
+class VerboseCursor : public sigc::trackable
 {
 public:
 	VerboseCursor (EditingContext&);