Initial whack at moving MIDI processing to the worker threads for
improved low-latency multithreaded processing...

REGRESSION:  Audio item #1 in a region doesn't fire off correctly,
presumably this has to do with the order that finish_time_params()
is being called in now...

UNTESTED:  External MIDI devices
Don't process external MIDI twice.
A fairly huge re-write of the audio sequencing engine...  First impressions
are that it fixed everything without breaking anything else, but more
regression testing is in order...
Fix a crash when entering/leaving an area with no region...
Fix all of the problems with the new audio/MIDI engine (I think).