Draft: Sound-related lua binders

[namishere]

This adds: SoundStream: holds a (currently disabled in lua) SoundActor and info about volume, its target volume, fade rate, if it's playing, and a reference to its owner SoundStreamSet SoundStreamSet: holds all music-related streams (one base, six more for the layers), info about the global music volume/target volume/fade rate/playing, as well as a bitset of currently active layers. MusicEntry: holds info from music.xml (id, filepaths, layer mode, layer fade rate, multiplier, looping) and a list of MusicLayer. MusicLayer: holds filepaths and a volume multipler.

Unfortunately, I have so far been unable to make SoundActor work. Calls made with the current binders will either do nothing, do the wrong thing, or crash.