Missing switch for clock source

[MuellerSeb]

Thanks for this project, it actually made me buy a 18i6 ;-)
One missing thing is a switch for the clock source (internal/ADAT/SPDIF). For now I can use the alsamixer to select it, but I think this could be done with a few lines of code.

Unfortunately I don't have time at the moment to get familiar with the code, so this is just for the record. Maybe @x42 it is quite simple for you to add this selector.

Thanks for all your great tools.
Sebastian