Add UI sound type and suitable API to sound manager

[historic_bruno]

We need a way to play UI sounds (e.g. button clicks #948) with a separate gain so the player can mute them independently if they choose. They can't really be lumped into music, ambient, or action sounds.

There should be simple C++ and JS APIs for playing these sounds, and it should work independently of the simulation. In particular, GUI code shouldn't have to manage the sound data, but it should be cached in the sound manager for efficiency. A single function like PlayUISound(filename) would be nice.

[stwf]

This should now be included in the latest svn revision