MPD: Make album adding a bit more greedy.

This fixes a regression with 4b7c035 where the album would be added without an
artist being specified. This also adds a helper method, no documentation as
this code is likely to be rewritten prior to next release.
StreamHandler: If the headphones plug is pulled, turn streaming off.

If the headphone plug is pulled, it's a likely expectation that the user no
longer wishes to stream.
StreamHandler: If the stream is stopped during buffering, don't play.

This should fix an inconsistency bug where if a stream was playing, and the
stream would be stopped by the checkbox during buffering, the stream would
continue playing and there would be no way to control stopping it without
killing the instance.

This fix is essentially the same thing as before, only slightly more strict.