Todo for xml interface 'ConTeXtXML' extension:
* add CDATA support for the content of <'context>', <'xmlcode>', <'texcode>'.
* (maybe) add a dedicated editing page with split fields
* document the new stuff (ref pages, variants)
* implement merging with new interface xml releases
<precontext source="yes"><\starttypingSome demo stuff, including a bad >entity;and a wrong </endtag>\stoptyping</precontext>