* 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?"
First pass at a level storage format to save and restore maps.
To save a map: press F12. It takes a screenshot PNG into the
screenshots/ folder and outputs a map JSON in the working directory.
To restore a map: "go run cmd/doodle/main.go map.json"
