Clean up dependencies and make things optional when possible

[zbanks]

• [ ] Can we replace PortAudio with QtMultimedia? (Or offer it as a fallback, the latency is really bad)
• [ ] Can we add a PortAudio shim to support both pre-V19 and "2.0" versions of the library?
• [x] rtmidi should be optional (esp. given it's not hooked up right now...)
• [ ] rtmidi can be a submodule & statically linked against
• [x] libmpv should be optional (pending merge of the mpv branch)

No-gos:

• libfftw3 could even be optional, falling back to a dumber ODF fn in BTrack. Need it to generate the spectrum anyways.
• Why do we need libsamplerate? BTrack

[ervanalb]

RtMidi now can't quite be compiled out because of application.qml

[ervanalb]

(fixed that)