gdk/wayland: Convert selection mimetypes back to atoms, if needed
Commit 0c1ea922 took care of converting STRING/UTF8_STRING to mimetype strings when letting selection targets known to the outer world through wl_data_source/zwp_primary_selection_source_v1, but it missed the conversion of those mimetypes back to the old atom strings, depending on the application, the mimetype counterparts might not be known or handled, so requests to paste from this app could go ignored.
Fixes: 0c1ea922 - wayland: Translate STRING/UTF8_STRING selection atoms to mimetypes
Closes: #5397 (closed)