Improve clustering using directory structure if no existing album tags

These guessed values are only used for grouping and for cluster title / artist and are not added to file tags.

I plan to adjust Lookup (once this is merged and we have some feedback) to use the cluster title / artist in the search criteria if not present in the file tags.

See http://forums.musicbrainz.org/viewtopic.php?id=4954 for problem report.