ENH: fix pb with deleting VTK widgets. If a widget (vtkInteractorObserver) ever requested a cursor shape to the vtkObserverMediator, this one would store a pointer to this widget inside its internal map. After deleting that widget, the next time another widget would request a shape, the map would be iterated over and an access to the (invalid) pointer of the widget that was just deleted is made, causing a crash. Fix that by making the widget will ask the mediator to remove every trace itself before deletion.