Investigate making QDomNodeList::It bidi-only

QDomNodeList currently implements an indexed iterator, basically calling item(n). This allows it to be random-access, but requires the QDomNodeList to linearize itself (maybeCreateList(), noexceptLength()), which takes time and extra memory.

An alternative is to use a sentinel end-iterator and have the QDomNodeList::const_iterator iterate the underlying data structure (a tree) directly, as per QDomNodeListPrivate::forEachNode(). Then this would be more akin to firstChild()/nextSibling() iteration, which does not require linearization of the list.