title: Where do you set preferredDisplayMode for UISplitViewController?
body: <p>I'd like to create a "slide-out-menu" for a portrait application on iOS 9 that allows pan gestures to switch between modes.</p>
  
  <p>I would like it to switch between the modes "Hidden - The secondary view controller is displayed onscreen and the primary view controller is off screen" and "Overlay - The secondary view controller is onscreen and the primary view controller is layered on top of it."</p>
  
  <p>According to <a href="https://developer.apple.com/library/ios/documentation/UIKit/Reference/UISplitViewController_class/index.html#//apple_ref/occ/instp/UISplitViewController/displayMode" rel="nofollow">Apple's documentation</a>, I need to set preferredDisplayMode for the UISplitViewController. Can somebody tell me where to modify this property?</p>