The Buttons can now be managed by a ui.Supervisor and be notified when
the mouse enters or leaves their bounding box and handle click events.
Current event handlers supported:
* MouseOver
* MouseOut
* MouseDown
* MouseUp
* Click
Each of those events are only fired when the state of the event has
changed, i.e. the first time the mouse enters the widget MouseOver is
called and then when the mouse leaves later, MouseOut is called.
A completed click event (mouse was released while pressed and hovering
the button) triggers both MouseOut and Click, so the button can pop
itself out and also run the click handler.
