title: Handle when AVPlayerViewController goes full screen
body: <p>My application is implemented just for the portrait mode. With the MPMoviePlayerController player there were notifications like MPMoviePlayerWillEnterFullscreenNotification and MPMoviePlayerWillExitFullscreenNotification. I could then adjust the supportedInterfaceOrientationsForWindow in the app delegate. It was working well. </p>
  
  <p>Is it possible to handle any property change using KVO with the new player/item so that I can detect the moment when it goes full screen without a private api? I tried to observe the bounds, the presentation mode. But nothing has changed when the Full Screen button was pressed. Why is it not possible to handle the buttons of the AVPlayerViewController?</p>