fixed mixed content elements and CDATA attribute lists in dtd, changed <def> semantics

* fixed [mixed content elements](](http://www.w3.org/TR/2008/REC-xml-20081126/#sec-mixed-content) syntax according to W3C Recommendations
* fixed [attributes with character data type](http://www.w3.org/TR/2008/REC-xml-20081126/#NT-StringType) according to W3C Recommendations
* PROBLEM! changed semantics of <def> because of it being a mixed content element

I understand the main point of the old <def> definition, is that if an article definition contains character data, then it can't contain a nested <def>. I couldn't express this as a DTD element constrain, though. Any ideas?