Exif tags Exif.Image.Artist, Exif.Image.ImageDescription, and
Exif.Image.Copyright were not being synchronized with their Xmp and
Iptc.IIM equivalents in the metadata-editor.

This commit adds synchronization of reading and writing of the metadata.

We adjust the struct for equivalent_metadata_tags by removing unused
fields and adding a field that points to the equivalent Exif tag in
a new struct called exif_tag_info.

On loading of the metadata tags we now also check a limited set of
Exif tags. If no value was set yet, we use the Exif value, or else
we check to make sure both had the same value. If not, print a
warning in the terminal.

On saving of metadata, we now also save to the marked Exif tags, or
if the value is NULL, we remove the tag.

In passing, we also fix two memory leaks by freeing with g_strfreev
temporary string lists.