We didn't always set an implementation, when the foreign toplevel wasn't
found, and when the importer tried to set the parent-child relationship,
the implementation was missing and we'd crash in wl_closure_invoke() in
libwayland-server.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2834>
This replaces the v1 implementation, which is now renamed to
legacy-xdg-foreign. Both implementations use the same data structures
internally, so that protocol version mismatches between
the importer client and exporter client don't fail.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2770>