Native MessageDialog: connect the new version of the clicked signal

After f212322a6ecb237d5fb53e56c6dafffb87e45f09 there was a warning on
connect, and it was not possible to dismiss a native MessageDialog
(for example on Android).

Change-Id: I12f62d5bdb61176ea2ff08c6be3731010cb47d45
Reviewed-by: BogDan Vatra <bogdan@kde.org>