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

This is a GNOME-specific key, but we honor it in Plasma as of
plasma/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.
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