We were not taking into account tags that can appear multiple times,
such as "keyword", they are handled by gexiv2 with the
get_tag_multiple() and set_tag_multiple() functions.

gimp_metadata_deserialize_text(): when deserializing our XML format,
check if a tag is already set on the metadata as "multiple" and if yes
retrieve it, append the new value and set it again.

gimp_image_metadata_save_finish(): take care of "multiple" values when
copying tags to new metadata created for saving.

This should preserve all values across an "import, edit, export".

Thing will still break when using the metadata editor, it doesn't
handle multiple values at all, but that code is very hard to
understand.