Additional MIDI backends

[OPNA2608]

RtMidi offers some additional MIDI backends that could be enabled with ease in pc/Makefile.

One is a dummy backend that serves as a fallback if no connection to any other compiled-in API endpoint could be established. It is enabled by passing -D__RTMIDI_DUMMY__.

The other is a MIDI endpoint for JACK, a real-time & low-latency daemon for professional audio tasks available on quasi all 3 big platforms. The RtMidi side of it can be enabled with -D__UNIX_JACK__. As the name implies the implementation is intended for Unixoid systems (macOS and Linux), I'm unsure how a Windows system using JACK could be supported.

For the latter, this should be achieved by letting the user decide at build time whether they want ALSA or JACK. Ideally, this would boil down to switching to a proper build system with dependency detection eventually, but for the start detecting an envvar / custom define and adding the required compiler arguments would work well.

Edit: Additionally, an in-app menu to select the preferred backend would be great to have.