[dtbook-to-zedai] Add MathML support

 * MathML islands in the DTBook are copied over to the ZedAI result
 * If MathML is present in the ZedAI, the feature is properly declared
 * Added XSpec tests for the new functionality
 * The built-in DTBook schema is removed, the schema from the dtbook-validator
   is used instead.
[zedai-to-html] Add MathML support