Extended attributes and import of meta-data

The format of gallery.list has been extended to allow multi-line
entries containing further attributes of photos. This can be used
to provide extra pieces of meta-data like geographical coordinates,
which are further propagated to cache files via gal-gen.

Also, gal-scan can be fed partial gallery.list entries on its stdin,
which makes it easier to import photos from external programs
including selected meta-data.

Finally, "gal-scan --add" can be used to add more photos to an existing
gallery.