core: Add volume arg to volume_changed() event

It was already called with the argument, and both the MPD and HTTP frontends
handled it/expected it. It was just the default implementation in CoreListener
that lacked the argument.
mpd: Trigger 'output' idle event on mute_changed()

This is required for e.g. ncmpcpp to detect that an enableoutput/disableoutput
command worked, making it possible to toggle the output back without restarting
ncmpcpp.