Dialogs/QButtonBox: the help button should not auto-dismiss the dialog

If a dialog shows a help button, it doesn't make sense that the dialog must be dismissed before the help can be shown, because then the help is no longer in context. So pressing help should emit buttonClicked(QAbstractButton*) but not accepted() or rejected(). Likewise in the QtQuick dialogs API, it should only emit a clicked signal with a StandardButtons enum value but not accepted or rejected, and should not close.