FR: Music Editor for FRED

It may be long overdue for one of these to allow mission designers full control and preview over their music, since the soundtrack system isn't very straightforward.
Currently, music manipulation is done entirely by .tbl or .tbm, but there's no way of previewing that.

Ok, so now I'm just going to rattle off a list of design points:

- Editor should be able to change every variable done in the .tbl (Sound tracks, Music tracks, measure numbers, samples)
- Editor should have a waveform viewer that shows how the tracks are sliced into measures
- Editor should have a preview system where the mission designer can arbitrarily fire off change-track events from a bank of radio buttons and see how the music transitions across each state.