use GTK_IS_RADIO_MENU_ITEM instead of comparing the type directly. This

use GTK_IS_RADIO_MENU_ITEM instead of comparing the type directly. This

Sat Oct 12 15:44:29 2002  Manish Singh  <yosh@gimp.org>

        * gtk/gtkitemfactory.c: use GTK_IS_RADIO_MENU_ITEM instead
        of comparing the type directly. This handles possible subclasses
        too. (Recommended by Tim Janik)