iOS: Add hooks to optionally get notification of viewDidAppear from root view controller

I have a need to sometimes present native screens outside of the libGDX framework, for example to allow for a login screen to an online service. When using the iOS backend, there is no way to hook into the root view controller's methods. This is required because you need to call presentViewController() after the root view controller has been pushed onto the view stack, i.e. in its viewDidAppear() method. If you call presentViewDController() earlier, for example in the application delegate's didFinishLaunching() method, you get a  "Unbalanced calls to begin/end appearance transitions" message.