Let UIKit automatically forward appearance methods

We don't have to manually forward the methods to be able to use
begin/endAppearanceTransition:animated:. Note that we do, however,
need to pay attention to the fact that the appearance methods are
called when the views enter the view hierarchy. So we need to make
sure to call begin/endAppearanceTransition:animated: after the
controller's view is loaded (as UIKit does), but before it's added
to the view hierarchy.