Add support for other MIDI-based clocks and MIDI output

[fwcd]

Currently midilink only supports Mixxx's non-standard MIDI note mapping and only in one direction (MIDI -> Link). It would be cool to support other MIDI clocks too, e.g. MIDI beat clock, ideally in both directions (would require us to generate a periodic beat output)

[fwcd]

This would be useful to map e.g. an external MIDI controller for resetting the downbeat too