Fix some clazy warnings
- Pass a context object as 3rd connect parameter
- QARG() argument signature is not normalized. Use QUrl instead of const QUrl&
- lambda capture 'this' is not used
- getter possibly mismarked as a slot, only added a KF6 TODO (does moving a method from Q_SLOTS to public/private: break BC?)
- C++11 range-loop might detach Qt container
- Missing reference in range-for with non trivial type QPointer
- Remove unused local vars
- slot return types need to be fully-qualified (KAuth::ActionReply instead of ActionReply)
Plus some other minor optimisations.
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