* 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.
* Add sound effect and music support to Doodle.
* Fix WASM build to use the 'null' sound driver for now.
* Add a Settings button to the main menu; UI for it is WIP.
Adds support for sound effects in Doodle and configures some for various
doodads to start out with:
* Buttons and Switches: "Clicked down" and "clicked up" sounds.
* Colored Doors: an "unlocked" sound and a "door opened" sound.
* Electric Door: sci-fi sounds when opening and closing.
* Keys: sound effect for collecting keys.
JavaScript API for Doodads adds a global function `Sound.Play(filename)`
to play sounds. All sounds in the `rtp/sfx/` folder are pre-loaded on
startup for efficient use in the app. Otherwise sounds are lazy-loaded
on first playback.
