Implement MouseGrab/MouseUngrab events

These events are similar to FocusIn and FocusOut events, except that they tell the widget "now you are going to get all mouse events" (== MouseGrab) and "you will no longer get any mouse events" (== MouseUngrab).

This is useful for widgets that keep mouse state (like an auto-repeat button) that need to reset that state when the mouse "moves" to another widget.