Proof of concept: advanced background music management

[FreeSlave]

A lot of mods used fmod for playing music. We haven't yet incorporated fmod in branch of any mod that originally used it. And these days we probably shouldn't do so. We should seek for a lightweight opensource alternative. @nekonomicon suggested we should try miniaudio.

This has a potential of improving the music experience in mods, e.g. not interrupting the music on changelevel.