Make it once again possible to compile KOffice with KOXML_USE_QDOM defined; this is not defined by default because it breaks kpresenter, kchart and kplato and it is slower, but it should be definable so we can check for errors in our dom implementation.
(And I'm sure there is one, since if I define KOXML_USE_QDOM, krita loads its documents with Qt45 again, which is broken if it's undefined, but I can now start to debug the problem again).

Note well: KoXmlNode has never been meant as a class you can extend with convenience methods.
Its API should be completely identical with the correspoding Qt class!
Convenience methods should be defined outside this class!

Also note: I have marked all places where applications use the aforementioned convenience methods with XXX!!!