StreamingService: Comment additions, cleanups and breathing space.

Add a few comments here and there, clean up some old comments and add some
space to increase readability.
StreamingService: Move coverart notification to it's own method.

Move the coverart stuff from showNotification() to it's own method. Also move
the MetadataEditor to the proper method, setMusicInfo(); this clears up a TODO
and should make things a little more clear for readability.