Have copy() only on QNode

It was previously both on QNode and QNodePrivate. The one on
QNodePrivate didn't give us much as it didn't eliminate the virtual of
QNode.

Change-Id: Ia1ea9dd831674109c84bd7005abe7fde3b1223fa
Reviewed-by: Paul Lemire <paul.lemire@kdab.com>
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>