* Add a tab bar to the top of the Palette window that has two
radiobuttons for "Palette" and "Doodads"
* UI: add the concept of a Hidden() widget and the corresponding Hide()
and Show() methods. Hidden widgets are skipped over when evaluating
Frame packing, rendering, and event supervision.
* The Palette Window in editor mode now displays one of two tabs:
* Palette: the old color swatch palette now lives here.
* Doodads: the new Doodad palette.
* The Doodad Palette shows a grid of buttons (2 per row) showing the
available Doodad drawings in the user's config folder.
* The Doodad buttons act as radiobuttons for now and have no other
effect. TODO will be making them react to drag-drop events.
* UI: added a `Children()` method as the inverse of `Parent()` for
container widgets (like Frame, Window and Button) to expose their
children. The BaseWidget just returns an empty []Widget.
* Console: added a `repl` command that keeps the dev console open and
prefixes every command with `$` filled out -- for rapid JavaScript
console evaluation.
