Aspect ratio support for iOS

• Recommended aspect ratio information is now returned by Letterbox view when calling its -animateAlongsideUserInterfaceWithAnimations:completion method. Applications can use this value as a hint about which aspect ratio is ideal for displaying the view.
• For iOS 10 and above, an NSLayoutConstraint category method provides simple methods to update aspect ratio constraints. On iOS 9, we recommend you either choose a fixed aspect ratio or use another strategy.
• The placeholder image fills Letterbox view entirely no matter the frame size.