Dialog: do not emit signals twice when a button is clicked

buttonClicked() is public API, whereas click(button, role) is called
when a button is clicked.  So there is no need to have buttonClicked()
connect to anything internally.

Task-number: QTBUG-40613
Change-Id: I25872d532ba77b4d708a2c25b31b6064fd5da599
Reviewed-by: Liang Qi <liang.qi@digia.com>