First and foremost, this patch implements the latest best practices in C++ (i.e. comparison operators are not member functions).
The main objective of this rewrite is to have these operators callable in slightly more general contexts (*this->asFontTag() == *rhs.asFontTag() yielded an error because no member function could be called). This feature is no more required.
The rewrite took place in the context of https://www.lyx.org/trac/ticket/12585. The first iteration was using more complex code to circumvent XMLStream in DocBook (see details in the bug above and in the mailing list), while this one uses XMLStream fully. The bug was due to font tags being considered equal when they were not, due to the comparison done only on the XML tag and not on the actual font change (XML attributes can complement the tag for various font changes).
Before, there was always a space after the tag, even when there were no attributes. Now, the space is output conditionally, so that the output makes more sense.
Before, one space too many for the tag name (title), casting doubt on whether the code had this space hard coded or not:
<!-- Output Error: Unrecognised tag type 'para' for 'title ' -->
After, no such space:
<!-- Output Error: Unrecognised tag type 'para' for 'title' -->
IGNORE is a typical placeholder for tags that should not be output, along with NONE. At some point, we should check if both are required, or if NONE is enough…
Actually output something when list item is empty. XMLStream discarded the sequence StartTag/EndTag (nothing in between).
New-line behaviour around term in description lists.
