Allow tagger script to access file metadata

Fixes PICARD-259 (and associated sub-tickets)

This moves the taggerscript evaluation to happen when a file is matched
(i.e. when a file is added to a track), where it has access to the both
the file metadata *and* the track metadata.

Because the track metadata no longer has the post-taggerscript data, the
preview of an unmatched track in the metadata box would be incorrect, so
I evalute the taggerscript there (on a copy of the data...) to handle
the special case.