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 (merged). 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.