Support new OS X Notification Center banners for systray balloon notification.

Since SDK 10.8 there is a new NSUserNotificationCenter provided by OS X. Any users' applications can post their notifications via new Notification Center. Every notification is appearing as a banner for some time then is passed into the system wide notification list.

We need to use this new feature of OS X in our Qt-based application via QSystemTrayIcon::showMessage(), but there is two limitation of current Qt API that sould be extented:

1. Several notification of the same application can be displayed in the Notification Center simultaneously. So we need to know which notification exactly was clicked by user in the messageClicked() signal.
2. All notifications can stay in the Notification Center even after its application was closed. So we need a way to discard notification at application exit, for ex.
3. According to the global system preferences, all notifications can be postponed and delivered later. So we need a new signal to know when the message has delivered.