title: Notification (not method) when Auto layout Trait Collection changes?
body: <p>Is there some kind of system wide notification when the Auto Layout Trait Collection changes in iOS 9+?</p>
  
  <p>I know the <code>UIViewController</code> method <code>traitCollectionDidChange</code> but I would like to handle the trait change in the App Delegate which does not have this method. </p>
  
  <p>The goal is to inform all (relevant) ViewControllers immediately when the change happens. <code>traitCollectionDidChange</code> is only called for the top most VC and when a VC becomes visible/active again.  </p>