This is one of the problems mentioned in issue #5863. This commit only fixes
our loading (and saving) of this metadata.
Certain iptc tags like "Keywords" can appear multiple times. We were using
gexiv2_metadata_get_tag_string to get the value but that always returns
the first value so we replaced that by gexiv2_metadata_get_tag_multiple.
(cherry picked from commit d0f67faf29)