[messages] is 'music' export needed?

@rigrig I've just been looking through the `messages` library and I see we still have `music` export with the currently playing track info in.

But as far as I can see, this isn't used by anything now? Any playing music is treated as a new item in the `messages` list (which `messagesgui` uses), and since we're only saving it on exit now, it's not a big deal for us updating it.

Or am I missing something?