Obtains a list of all toplevel windows known to GDK on the default
screen (see gdkScreenGetToplevelWindows).
A toplevel window is a child of the root window (see
gdkGetDefaultRootWindow).
WARNING: gdk_window_get_toplevels has been deprecated since version 2.16 and should not be used in newly-written code. Use gdkScreenGetToplevelWindows instead.