Add scene module.
Add stage module.
Add temporary dummy procedure.
Use the new scene/stage API.
Remove debugging statement.
Update animation demo.
Use stage agenda.
Update actions example.
Update coroutine demo.
Update font demo.
Rewrite stage variables.

Stage variables now act a lot like parameters and their initial values
are lazily evaluated.
Update action and animation example to use new stage variables.
Add observer to stage.
Remove stage local variables.

Use an the scene initialization thunk to return the needed game state
instead.
Add scene name field.
Export scene type tag.
Revert "Export scene type tag."

This reverts commit d1ecb21fca03cdd0e9360a4f1b88c7e05a9354e2.
Really export scene type tag.
Add event observer to scene.

Some convenient default events are provided to make quitting the game
easy for new developers.
Fix default key-down event.