* Added the Death Barrier to Play Mode to catch players from falling off
the map and then falling indefinitely, especially on Unbounded maps.
* The Death Barrier is set 1,000 pixels below the lowest point on your
map. If the player falls here they get a death message: "Watch out for
falling off the map!"
* Added cheat codes to change the default Player Character doodad, as a
way to force play as a different character (for levels which don't
specify a custom character):
* Play as Bird: fly like a bird
* Play as Blue Azulian: the cell
* Play as Thief: play as thief
* Reset to default (Boy): pinocchio
* The crumbly floor doodad was made 50% larger.
* New doodad: Small Key and Small Key Door. These work like the colored
doors and locks except each Small Key is consumed when it unlocks a
door. The door's appearance is of iron bars.
* The inventory HUD displays a small quantity label in the lower-right
corner of items that have a quantity, such as the Small Key. This is
done as a Canvas.CornerLabel string attribute on uix.Canvas.
* The "give all keys" cheat adds 99 Small Keys to your inventory.
* Take advantage of the new Window Manager feature of the UI toolkit.
* Move the MenuScene's "New Level" and "Play/Edit Level" windows into
stand-alone functions in new pkg/windows/ package. The 'windows'
package is isolated from the rest of Doodle and communicates using
config variables and callback functions to avoid circular dependency.
* MenuScene calls the window constructors from the new package.
* Add an "Options" button to the Menu Bar in the Editor Scene, which
opens the "New Level" window to allow changing the wallpaper or
bounding type of the level currently being edited.
* Move the cheat codes into their own file, cheats.go
