* Overhaul the clunky old alpha Edit Level/Doodad menu with a modernized
version featuring the new ListBox widget.
* The new level loader is a Window that can be spawned from anywhere instead
of on a dedicated MenuScene.
Updates to doodad scripts:
* Actor.IsOnScreen() checks whether an actor's visual sprite box is on-screen
in the level viewport. `Self.IsOnScreen()` will check for the current actor.
Other changes
* PlaySound() to deduplicate the same sound effect from playing at once.
