OriginalXmlProperties optimization

[elpaso]

Dramatic speed improvement in XML original properties storage.

The original implementation used a call to QDomNodeList::count() method
that calls QDomNodeListPrivate::createList() -> DETACH!

By using a while loop we prevent the detach and it's more than 10 times faster.

[nirvn]

@elpaso , I was planning to fix that one if you didn't address it, very good :)

This can be optimized a bit more:

• the original xml stuff is only ever used for invalid layers when the project is loaded, therefore you can skip the creation of this original xml altogether if the matching layer isValid()
• maybe more of a micro optimization but you could also avoid creating a whole DOM document and instead use a ptebuilt string within which you'd insert your XML node

[nyalldawson]

Manually backported to #40370