Mute/Unmute players

[ghalestrilo]

When 1 or more channels are selected, pressing 'm' should mute them.

IDEA: each plugin has a mute/unmute command combination (ie.: "mute $channel_name" for tidal, "$channel_name.mute()" for foxdot), and we send these commands to the repl based on selection (requires multi-select)

We'll need a variable syntax for this, but it seems inevitable right now - it will come up in different places.