* Add ui.Window to easily create reusable windows with titles.
* Add a palette window (panel) to the right edge of the Edit Mode.
* Has Radio Buttons listing the colors available in the palette.
* Add palette support to Edit Mode so when you draw pixels, they take
on the color and attributes of the currently selected Swatch in your
palette.
* Revise the on-disk format to better serialize the Palette object to
JSON.
* Break Play Mode: collision detection fails because the Grid key
elements are now full Pixel objects (which retain their Palette and
Swatch properties).
* The Grid will need to be re-worked to separate X,Y coordinates from
the Pixel metadata to just test "is something there, and what is
it?"
* 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.
