* src/glade-gtk.c, widgets/gtk+.xml: Added support for GtkCombo, GtkList, GtkCList
	  Also added support for the new "get_internal_children" backend function

	* src/glade-popup.c: Adjusted to make the "Add" submenu available
	  for any special container relationships (not just non-widgets).

	* src/glade-utils.[ch]: Added glade_util_purify_list()

	* src/glade-widget-class.[ch], src/glade.h: Added GladeGetInternalChildrenFunc and changed
	  glade_widget_class_contains_non_widgets for glade_widget_class_contains_extra