Missing options to control matrix channels

[manuelveith]

Hi,

I have in my setup some AUXes converted to a matrix.
This function seems to be missing completly, so I can't change levels for AUX, Sub and Master in a matrix (MTX).
Only the master channel level for the AUX (or MTX in this case) can be changed now.

[fmalcher]

This is currently blocked by the connection lib not supporting it yet: fmalcher/soundcraft-ui#38