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.
Showing with 242 additions and 44 deletions