libtaskmanager: also honor new XDG "SingleMainWindow" key

Merged Nate Graham requested to merge ngraham/plasma-workspace:support-xdg-thingy-too into master

This is the upstreamed version of X-GNOME-SingleWindow which is now a part of the XDG desktop file spec as of version 1.5; see https://gitlab.freedesktop.org/xdg/xdg-specs/-/merge_requests/53

cc @nicolasfella

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