响应事件传递nextResponder

由于项目比较大，页面多而且复杂，有时就需要从当前的responder通过nextResponder（一个甚至多个）找到深层次的VeiwController。

官方解释 

UIView implements this method by returning the UIViewController object that manages it (if it has one) or its superview (if it doesn’t); UIViewController implements the method by returning its view’s superview; UIWindow returns the application object, and UIApplication
returns nil.