This addresses issues with session-cleanup and region-cleanup in
some sessions.
The root-cause why some unused playlists were saved in the session XML
under <Playlists> and not <UnusedPlaylists> is not known.
Early 6.0-pre did incorrect reference counting, but also older sessions
had this issue. Perhaps due to ambiguities of matching playlists
by name in 5.x or session-format changes 3.x .. 5.x.
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