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.
