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.