initial jack-audio support

[lyrra]

Jack-audio support.
Enabled at build-time by selecting MUE_ENABLE_AUDIO_JACK=ON, ie no run-time selection support between jack/alsa.

• [x ] I signed the CLA

TODO:

• hangs jackd (which cant recover even after musescore is killed) (can't name client "default")
• outputs audio to jack, but it is butched (was counting frames vs bytes)

Try it out:

1. start jackd with MuseScores default 44100Hz:
   jackd -r -d dummy -r 44100
   (If using the dummy driver, record the audio using jack_capture).
2. start MuseScore
3. jack-connect musescore to jack's default output
   This is heusteristics that Mu3 did, but maybe should be handled by environment/setup managers such as Carla.

   $ jack_connect MuseScore:audio_out_right system:playback_2
   $ jack_connect MuseScore:audio_out_left system:playback_1
   

[igorkorsukov]

@lyrra Now the choice of the driver is made at the compilation stage, respectively, Jack support will not be included in the release builds.
Do you plan to add driver selection at runtime, to the settings?

[lyrra]

(squashed commits for cleaner history).

No plans so far on adding runtime selection support between jack and alsa.
Because that requires more in-depth understanding and perhaps UI-design.