fix that finishButtonClicked didn't come in every case

  - we misuse the cancel button as a finish button in the
    packagemanager/updater mode to have some more control if it is quited
  - now we connect the cancel button to the finishButtonClicked and disable
    the reject -> setCanceled connect for that moment

Change-Id: I45182da12a5652f193b20af52262852836e29979
Reviewed-by: Rainer Keller <rainer.keller@digia.com>
Reviewed-by: Karsten Heimrich <karsten.heimrich@digia.com>