Add support for cascaded mixers

[fmalcher]

It is possible to cascade two Ui24R mixers. As described in https://github.com/bitfocus/companion-module-soundcraft-ui/issues/71, the messages to the remote mixer differ from the standard ones:

// Channel 1 mute activated in aux 1 in HOST:
SETD^i.0.aux.0.mute^1

// Channel 1 mute activated in aux 1 in REMOTE:
@setd^i.0.aux.0.mute^1