Mark as single-instance using X-GNOME-SingleWindow key

This is a GNOME-specific key, but we honor it in Plasma as of
plasma-workspace!1309. I
have proposed upstreaming it as "SingleInstance" as part of the FDO
Desktop spec in
https://gitlab.freedesktop.org/xdg/xdg-specs/-/merge_requests/53.

Until that is merged, we can just use the GNOME key which works just
fine.