The memory selection source was only providing the "text/plain" or the
"text/plain;charset=utf-8" mimetype, but not "STRING" or "UTF8_STRING",
which some X11 clients, like wine, are looking for. This was breaking
pasting from the clipboard in wine applications.
Fix this by adding those targets when they are missing and the selection
source provides the corresponding mimetypes.
https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1369