Contract of PartialViewContext doesn't give enough freedom for JSF components libraries to override it
partially. Example: simple addition of "extension" element (or any other that RI's implementation doesn't
do, but component libraries may need) requires full re-implementation of PartialViewContext class, thus
making co-existence of different libraries almost impossible. If some AJAX-updated component outputs
"extension" element, then it appears inside "update" element causing malformed XML error on the client,
so this approach doesn't work.