add API to AudioBackend to allow discovery of possible MIDI I/O options
add JACK backend support for MIDI option discovery
basic use of new backend MIDI options
fix up parsing of send gain MIDI bindings so that they actually work
add API to get current MIDI option from an audio/MIDI backend
add API to get current MIDI option from an audio/MIDI backend
more fixes and rationalizations for MIDI option handling in the audio MIDI setup