Some X11 clients still rely on the traditional urgent/demand-attention hints instead of notifications to request the user's attention. Support these by adding a visual indication to the corresponding buttons, based on the visual indicator in libadwaita's tabs. Closes: https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/issues/543 Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/366>