Replace GTK_NO_CHECK_CASTS by G_DISABLE_CAST_CHECKS.

Replace GTK_NO_CHECK_CASTS by G_DISABLE_CAST_CHECKS.

        * docs/debugging.txt: Replace GTK_NO_CHECK_CASTS by
        G_DISABLE_CAST_CHECKS.

        * gtk/gtkaccelgroup.c (gtk_accel_group_from_accel_closure,
        gtk_accel_group_query, gtk_accel_group_find,
        gtk_accel_groups_from_object): Document.

        * gtk/gtkaccellabel.c (gtk_accel_label_set_accel_closure): Document.

        * gtk/tmpl/gtktypeutils.sgml: Replace GTK_NO_CHECK_CASTS by
        G_DISABLE_CAST_CHECKS, fix some markup and add stuff.

        * gtk/tmpl/gtkaccelgroup.sgml, gtk/tmpl/gtkwidget.sgml: Additions.

        * gtk/gtk-sections.txt: Make gtk_accel_group_get_type private.