Allow the play queue to be sorted; not playlists for now due to complications...

Allow the play queue to be sorted; not playlists for now due to complications in the implications (what does sorting do to the underlying list).

Relates to issue #120 - a simpler patch than what is discussed in this issue, to avoid controversy.

95% of credit goes to Sergey Rudchenko - I based the patch on his work (originally posted in https://bugs.launchpad.net/rhythmbox/+bug/76449).