Add <NavigationEvents/> component (#4188)

* add NavigationEvents

* expose TabsWithNavigationEvents in lib entrypoints

* Add NavigationEvents example in playground

* Add NavigationEvents example in playground

* Add NavigationEvents tests

* Add NavigationEvents Flow declarations

* remove useless NavigationEvents constructor

* NavigationEvents => make tests more readable by avoiding beforeEach callback

* fix flow test error by adding <any, any> to React.Component

flow/react-navigation.js

@@ -557,6 +557,21 @@ declare module 'react-navigation' {
     navigationOptions?: O,
   }>;
 
+  /**
+   * NavigationEvents component
+   */
+
+  declare type _NavigationEventsProps = {
+    navigation?: NavigationScreenProp<NavigationState>,
+    onWillFocus?: NavigationEventCallback,
+    onDidFocus?: NavigationEventCallback,
+    onWillBlur?: NavigationEventCallback,
+    onDidBlur?: NavigationEventCallback,
+  };
+  declare export var NavigationEvents: React$ComponentType<
+    _NavigationEventsProps
+  >;
+
   /**
    * Navigation container
    */