Update Xhtml backend inside `tuneup' functionality:
    
        - Stop using `ul' and `li' definitions to build the list of
          content.  Instead, start using `dl' and `dt' definitions. The
          same used in Docbook output.
    
        - Add support for classes and other definitions inside heading
          definitions (e.g., <h1 class="title">...</h1> is now possible).