Make shuffle mode more clever

When the user enables shuffle mode, we should generate a shadow playlist order that has been shuffled and play from that instead. Adding new tracks should add them to the "unplayed" part of the playlist. Deleting a track should remove it from the playlist.

When the end of the playlist is reached, it should stop.

From Dr. O:

> and this is why people complain about winamp proper's shuffle handling at times :wink:
as we've been talking about it recently as some seem to get repeats when shuffle is on when others don't & it works off a pre-generated shuffle list (with removals taken out & additions added into the part that's remaining to be played)