StreamingService: Rename a couple of variables for readability.

Rename resumeStreaming() to beginStreaming() and oldStatus to prevMpdState
to improve the logical naming. They make more sense to me, I hope they do
for you as well.
StreamingService: Move all stubs to the end of the file.

Take the stubs out of the body of the code to increase readability.
StreamingService: Remove large nested blocks in stateChanged().

Removing large nested blocks improves readability.
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.