Change klipper clear dialog to warningContinueCancel - doesn't save if No answered
Previously the "clear clipboard history?" dialog used KMessageBox::questionYesNo which if "don't ask again" is ticked then saves the answer even if No was answered. Changes it to KMessageBox::warningContinueCancel which doesn't do that, and is more appropriate for the kind of question we're asking.
BUG: 336053