* Fix the EditorUI not showing the correct palette baked into the level
and only showing the default. This was tricky because the palette UI
can only be configured at setup time but not updated later.
* Add a new default palette for the Blueprint theme. Blueprint has a
dark background, so the palette colors should be bright. This palette
is chosen when you start a map with the blueprint wallpaper.
* Add a background Canvas to the MenuScene. In the "New Level" screen,
the background canvas will update to show the wallpaper settings
you've chosen as a preview of the level theme you're about to create.