### old todo comments -- I don't know if they still apply...
If the document dies, remove all pointers to it from children
which can not be deleted at this time.
If a node dies and has direct children which can not be deleted,
then remove the pointer to the parent.
createElement and friends create double reference counts.
/* ##### new TODOs:
Remove emtpy emthods in the *Private classes
Make a lot of the (mostly empty) methods in the public classes inline.
Specially constructors assignment operators and comparison operators are candidates.
/** ### Qt 5:
* If we still have QDom, optimize this so that we only declare namespaces that are not
* yet declared. We loose default namespace mappings, so maybe we should rather store
* the information that we get from startPrefixMapping()/endPrefixMapping() and use them.
* Modifications becomes more complex then, however.
* We cannot do this during the Qt 4 series because it would require too invasive changes, and
* hence possibly behavioral changes.