QSystemTrayIcon: be able to connect to balloon message's closeButton clicked() signal.

When showing a message with QSystemTrayIcon, a notificationis sent if the user clicks the message. However, if the messaage is not received there is no way to tell the difference between "the user has not seen the message and the message has timed out" and "the user has closed the message after having seen it".
It would be useful if it were possible to connect to the balloon message's closebutton clicked() signal.