Initial whack at making MIDI in devices fully modular...  Compiling,
but definitely not working yet...
Multiple MIDI devices closer to being functional...
More progress on multiple MIDI devices...
More work on multiple MIDI devices at the same time
Update the device dialog to facilitate enabling multiple MIDI devices
Get the application compiling again...  (albeit, it immediately SEGFAULTs..)
The application can now run, and only crashes on the assert()'s I left in
there to remind myself that it's not finshed yet...
Hellz yeah!  Successfully configured and played 2 tracks at the same time
using my Arturia Microbrute (as a MIDI controller) and my M-Audio Oxygen49.

There are still some issues and a few regressions of things that rely
on external MIDI to sort out, but the vast majority of the work is done.
Code defensively for some potential failure scenarios with the new MIDI
devices...
Remember MIDI routing between sessions...