Metronome function

[KohlJary]

Would be useful as a practice aid to be able to set a metronome to send to the audio out on built in audio units. Would involve setting up a MIDI clock system internally, and ideally being able to use the MIDI in for a clock source as well. Having that in place would also open up a lot of possibilities for added effects, like arpeggiating the drone string on a certain beat value and the like. I can take a look at setting this up if it's something that you think would be useful to have in the codebase.