New properties are added to EventData for Supervisor events:
* Widget: a reference to the widget which is receiving the event.
* Clicked (bool): for MouseMove events records if the primary button is pressed.
* func RelativePoint(): returns a version of EventData.Point adjusted to be
relative to the Widget (0,0 at the Widget's absolute position on screen).
Other changes:
* Destroy() method for the Widget interface: widgets that need to free up resources
on teardown should define this, the BaseWidget provides a no-op implementation.
* Window.Resize() will properly resize a Window.
* Window.Center(w, h int) to easily center a window on screen.
