Commit 840378ae68 changed the code to use XmbTextPropertyToTextList()
instead of gdk_text_property_to_utf8_list_for_display(), but didn't
take into account that the replacement returns text in the current
locale's encoding, while any callers (rightfully) expect UTF8.
Fix this by converting the text if necessary.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/227
(cherry picked from commit 76abe87090)