* Added `BoxSize()` to Widget that reports the full box size including
borders and margin.
* The Frame uses the `BoxSize()` of widgets to position them.
Reintroduces some padding issues (boxes on the GUI Test stick out of
bounds a bit) but is on the right track.
* Renamed `Padding` to `Margin` on the Widget object, since the Margin
is taken into consideration along with Outline and Border in computing
the widget's BoxSize.
* Restructured the Label widget to take a Text or TextVariable property
and the font settings (render.Text) are in a new `Font` property.
