Add support for MIDI devices

[neilenns]

Is your feature request related to a problem? Please describe.
There are lots of midi devices out there that are essentially big boxes of buttons, encoders, and switches. The new Joystick feature laid the foundation for supporting devices like this. A couple of people have asked for MIDI support in the Discord now.

Describe the solution you'd like

Buttons/encoders/etc. on midi devices show up as inputs in MobiFlight
LEDs on midi devices show up as outputs in MobiFlight (is that even possible?)

Additional context

Here's a C# library that looks useful: https://melanchall.github.io/drywetmidi/articles/devices/Overview.html