Added mute button and key binding.

Either an M or - is displayed after the sink name. Pressing 'm' mutes/unmutes the sink.

I refactored the existing volume specific set/get function names to get_/set_volume and just copied the cache code to my get_/set_mute stuff. It is probably a good idea to make a common structure for this if more is added but this will do and cause less changes to be made.