**** In rng-valid, instead of using modification-hooks and
insert-behind-hooks on dependent overlays, use same technique as nxml-mode.
-**** Port to XEmacs. Issues include: Unicode (XEmacs seems to be based on
-Mule-UCS); overlays/text properties vs extents; absence of
-fontification-functions hook.
-
*** Fontification
**** Allow face to depend on element qname, attribute qname, attribute