MPDPlaylist: Add a crop method to the playlist.
MPD.java: Don't change MPD state when replacing on the playlist.

While playing if 'Add, Replace and Play' or 'Add and replace' are selected,
don't stop, clear, add and play. Rather than invoke a MPD state change, move
the current playing playlist queue item to queue postition 0, then remove the
remaining playlist items, add the replacement playlist queue items,
select the next playlist item, then remove song 0. It occurs in the same
amount of time and the play interruption is minimized. This behaviour causes
a track change listener emit rather than state change.
NotificationService: Don't check for empty playlist on pause.