Port to new KNS JobEngine
Draft because it needs polishing, including upstream in kns. See frameworks/knewstuff!246 (merged).
This is based on top of the work/apol/qt6 branch for obvious reasons.
Biggest advantage is that we get to drop the pending response property that was the corner-stone of the KNSBackend behaviour and the reason of most of its bugs.
Edited by Aleix Pol Gonzalez