Speed up deleting many items from playlist.

Originally removing 10000 items took me 55s.
Following changes decreased the time to 4.3s.

- Improve Status widget
- Use references for tracks from PlaylistItems. Updating the reference
count for tracks took a long time.
- Use references also for the playlist functions. Since the calling
function already has the shared pointer it's impossible that
it can run out of scope.
- Unsubscribe after the removals have been done since we have
a smaller list to check.