MIDI support for VSTi and MIDI Beat Clock

This series adds MIDI support using the PortMidi cross-platform MIDI library.  It enables two features:

1. VSTi instruments that take MIDI input and/or produce MIDI output.
2. MIDI Beat Clock support for synchronizing external applications and devices to the jam tempo.

Tested with S-Gear (patch switching), TruePianos (VSTi instrument), and SooperLooper (MIDI Beat Clock slave).  Tested MIDI I/O on Windows, Mac, and Linux.