_NET_WM_STATE_FOCUSED is a new _NET_WM_STATE hint which allows us to
implement a meaningful GDK_WINDOW_STATE_FOCUSED under X11. If the window
manager doesn't support this hint we keep GDK_WINDOW_STATE_FOCUSED set since
that is what gtk+ implicitly assumed historically.
https://bugzilla.gnome.org/show_bug.cgi?id=661428