wayland: Translate STRING/UTF8_STRING selection atoms to mimetypes

wayland: Translate STRING/UTF8_STRING selection atoms to mimetypes

Since the GDK API layer in 3.x is considerably tied to X11 details,
it makes sense to provide this bit of translation in case the app
uses these exclusively and does not use actual mimetype strings.
Provide this bit of translation of X11 selection atoms to mimetypes.

While at it, use the right zwp_primary_selection_source_v1 API with
primary selection data sources. From the looks of it, this just
happened to work.

Closes: https://gitlab.gnome.org/GNOME/gtk/-/issues/5217