Index media file usage in the metadata index and use it in ft_mediause()
Remove search_references() and the refshow configuration option

The refshow configuration option wasn't used as described anymore
already in the latest release and after the introduction of the media
usage index the parameter is also no longer relevant for internal
optimization. The only place where it was still used is the no longer
used search_references()-function which is removed here, too.
Merge pull request #244 from splitbrain/mediause_index

Index media usage data