* Button: do not call MoveTo inside of Compute().
* Label: do not call MoveTo inside of Compute().
* MainWindow: add OnLoop callback function support so you can run custom code
each loop and react to the event.State before the UI updates.
* Supervisor: locate widgets using AbsolutePosition() instead of w.Point()
to play nice with Frame and Window packed widgets.
* Widget interface: rename Adopt() to SetParent() which makes more sense for
what the function actually does.
* Window: set itself as the parent of the body Frame so that the Supervisor
can locate widgets anywhere inside a window's frames.
Frame packing fixes:
* Widgets with Expand:true grow their space with ResizeAuto to preserve the
FixedSize() boolean, instead of being hard-resized to fill the Frame.
* Widgets that Fill their space are resized with ResizeAuto too.
* Labels don't expand (window title bars, etc.)