Refactor MIDI library inclusion

[mrikola]

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.

[HunterZ]

I don't have any strong opinions on this, but here are some thoughts as the person who contributed native MIDI support:

• The upshot of ADLMIDI is that it works for everyone out of the box, even if it sounds the worst.
• Windows has a General MIDI software synthesizer built-in that the native MIDI support will use by default on that platform.
• Linux native MIDI support was only tested in a VM, and it takes some effort to set up a software synthesizer that works with it.
• I wasn't able to attempt native MIDI support for Mac, so it's not even an option there unless someone else added it.
• Ideally they should probably all be optional, and the game should figure out which ones got compiled in.