When setting the focus to NULL, clear the existing focus chain so that

When setting the focus to NULL, clear the existing focus chain so that

Tue Mar  5 19:19:19 2002  Owen Taylor  <otaylor@redhat.com>

        * gtk/gtkwindow.c (gtk_window_set_focus): When setting
        the focus to NULL, clear the existing focus chain so
        that when we focus into the window again, we start
        at the beginning. (Pointed out by Tim Janik.)