[Development] Replace QtXml backend

[Gerhard Scheikl]

> Did you consider using QXmlStreamReader/Writer instead of the old QXml?
> These classes are fully compliant, well tested and very fast. The only
> drawback is that there is no equivalent to the QDom* classes, but in most
> cases you’re better off parsing directly into your own data structures
> anyway.

Hi

Currently, our main problem is that the XML validator goes into an infinite 
loop and therefor out of memory.
Besides that, the validation is really slow.
(around 20 times slower than with xerces)
Unfortunately, for our use case, SAX parsing is not an option.

Best regards
Gerhard