* 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.
CheckButton is a generic component based on Button that additionally
takes a *bool variable to manage. When the CheckButton is clicked or
unclicked, it will toggle the bool var and its border style will "stick"
in or out depending on the state.
Checkbox is a Frame widget that wraps a CheckButton and another child
widget, such as a Label. Interacting with the child widget will forward
all of its mouse events to the CheckButton, so that the Label could be
clicked instead of just the box itself.
