[Request] Add mute state variable of channels and busses

[Wiethoofd]

It could be useful to have the '(un)muted' state of a channel or bus as a variable to display as text on a button, not just the visual feedback.

The OBS companion module has this for audio sources; the feedback state having the textual representation has come in handy more than once.

[istnv]

You can also add 'text' to the button via the muted feedback.