1. TXMLEngine class (ROOT xml parser) in addition to normal xml nodes now can parse
   xml processing instructions http://www.w3.org/TR/1998/REC-xml-19980210#sec-pi with syntax
      <?something attr="value" attr2="value2" ... ?>
   and xml comments http://www.w3.org/TR/1998/REC-xml-19980210#sec-comments with syntax
      <!-- my comment string -->
2. Comment line and style sheet definition (one special case of xml processing instruction)
   can be inserted manually by user with any text/xml editor or with new methods of TXMLFile
      Bool_t TXMLFile::AddXmlComment(const char* comment);
      Bool_t TXMLFile::AddXmlStyleSheet(const char* href, const char* type, ...);
3. Arbitrary text line can be inserted on the top of xml file with call:
      Bool_t TXMLFile::AddXmlLine(const char* line);
   Now this line can contain either comments or xml processing instruction, while
   anything else will not pass xml syntax check.


git-svn-id: http://root.cern.ch/svn/root/trunk@16434 27541ba8-7e3a-0410-8455-c3a389f83636