Keep GtkPlug in sync with the global list of toplevels.

    Keep GtkPlug in sync with the global list of toplevels.
        Patch by Federico Mena Quintero

        * gtk/gtkwindow.h:
        * gtk/gtkwindow.c (_gtk_window_set_is_toplevel): New internal
        function used when a GtkPlug parents/unparents itself by an
        in-process GtkSocket.  This keeps the plug's GTK_TOPLEVEL flag in
        sync with the global toplevel_list.

        * gtk/gtkplug.c (gtk_plug_set_is_child): Call
        _gtk_window_set_is_toplevel() to keep the toplevel list updated,
        instead of just setting/unsetting the GTK_TOPLEVEL flag.

svn path=/trunk/; revision=22191