Convert *itemsoverwrite* functions into theme functions so they could be overridden

While using this module, in some cases I need ‘nolink’ and ‘separator’ items to be rendered in totally different way than it is done in special_menu_itemsoverwrite_menu_item_link and special_menu_itemsoverwrite_menu_item functions. This patch converts these transparently into regular theme functions so they could be overridden by a custom theme.