As it is not allowed to called gtk_extended_layout_get_desired_size()
on a GtkWidget directly; gtk_widget_get_desired_size() was really an
ambiguous api. This patch removes the added GtkWidget api, calls
the appropriate sizegroup code which in turn envokes the interface
vtable, this patch also accordingly makes GtkWidget a prerequisite
of GtkExtendedLayout (the api doesnt work for cell renderers anyway,
patch comming...).