libtaskmanager: also honor new XDG "SingleMainWindow" key
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
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