Simplify KCMultiDialog
- We do not need Q_Q/Q_D, since we killed the KSettings stuff
- As the KF6 todo stated, unify one overload
- Remove unneeded "virtual" keyword from p-impl
- Remove Q_PRIVATE_SLOT statements, we have a PMF connect now
I want this to get in before finishing my large refactor/import of the KCM classes.