Move initial music load to a separate thread.

I ran into all the problems one might expect from adding threading to an
old codebase but this variant seems pretty stable.

The benefit is that the heavy I/O with large music libraries is now off
of the GUI thread. So even though it will still take awhile to load all
music until I fix the bug(s) with using cached tags, at least the
application itself will be responsive while it loads audio.