Fix entity update sequence

Levels should no longer start with a brief flash of invalid data, caused
by calling draw() before update().
Add sound effect code
Add sound effect data
Add code to trigger music
Add music files
Add track annotations to levels and the script
Remove treasure glyphs from UI
Fix bug with step sounds
Adjust volume of music and sfx
Fix per-SFX volume adjustment
Adjust individual SFX volume
Fix build on Linux