Refactor MIDI library inclusion

Current situation:
- ADLMIDI and Fluidsynth-lite are bundled in with the code
- Native MIDI support was added after the two
- ADLMIDI mostly sounds like crap
- Fluidsynth quality depends on soundfont; there's a really good one in the works over at https://www.systemshock.org/index.php?topic=10927 but the download clocks in at 350MB+

Target situation:
- Native MIDI is the baseline
- Fluidsynth is an optional nice-to-have for customization purposes
- ADLMIDI is the fallback if all else fails

Comments welcome as to which MIDI libraries should be included and how.