[Linux] Improve the I/O configuraton, split MIDI and audio setup

Hi,

currently MuseScore only allows to select one system at the time for both MIDI I/O and audio output.

IMHO it would make sense to split the audio configuration from the MIDI configuration.

For example is not unusual on linux to use the ALSA sequencer interface for MIDI I/O but still use pulseaudio for audio output, AFAICS this is currently not possible with MuseScore.

Thanks,
Antonio