* added method for getting the currently visible screen ID. Can be useful to decide if logic should be applied according to screen visibility, for example: set buttons/title dynamically, or even determine that you don’t need to show the same screen again if it’s the current one that’s showing

* Implement getCurrentlyVisibleScreenId on Android

Can be used statically:
Navigation.getCurrentlyVisibleScreenId() - returns the unique screen
instance id

With a navigator instance:
await this.props.navigator.screenIsCurrentlyVisible() - resolves a
promise that checks if the current screen is visible

* Add screen visibility listener which can be registered globally

API
new ScreenVisibilityListener({
    willAppear: ({screen, timestamp}) => console.log(`Displaying screen ${screen}`),
    didAppear: ({screen, timestamp}) => console.log(`Screen ${screen} displayed in ${Date.now() - timestamp} millis`)
  }).register();

todo
* Add timestamp to modal, resetTo
* implement unregister
* implement iOS

* Add global screen visibility listener to example app

* Think I better assign a value to emitter

* Send timestamp in resetTo

* Moved listener to library, it can be deleted from the app

* Add push and resetTo buttons to pushed screens

* Send appear/disappear events after animation ends

* Pass pop timestamp from js

* Send timestamp in popToRoot

* Add popToRoot button in push screen

* Add few navigation buttons to modal screen

* Pass timestamp to tabs

relevant only to first/initial tab

* Update timestamp when navigating between BottomTabs

* Send screen visible events after modal dismiss

* log willDisappear and didDisappear

* Report both start and end timestamps

* Implement unregister

* Send NavigationType parameter in visibilityEvent

This indicates the type of the navigation command that triggered the visibility change

* Rename pushInitialScreen to more explicit name

* Fix duplicate initialScreen visibility events

* Update screen visibility log statement

* Show popToRoot in modal screen only if screens were pushed

* implement global screen event for iOS

* fix lint issues

* Set to use old bridge for compatibility with 0.30+

* Fixed crash occurring when tabBarBackgroundColor or tabBarButtonColor is not supplied

* Adds setStyle property to index.js

* Adds native code for setting style on RCCViewController

* Fixes setStyle function

* Fixes setting style on navigator

* Adds some styling to tab bar controller and fixes some bugs

* Adds ability to set modal presentation style

* Makes sure autoAdjustScrollViewInsets isn't propagated through when push another view controller

* Changes import to reference controllers rather than react-native-controllers and fixes setStyle mutating colours

* Fixed to work with RN 33

* Fixes updating title font styling with new RCCTitleViewHelper logic

* Fixes subtitle being shown

* Adds extendedLayoutIncludesOpaqueBars to navigation styling

* Adds ability to add shadows to text in navigation bar / tab bar

* Fixed merged ReactGateway.java file

* Fixed issue with rightButtons getting ignored in preference for empty Screen buttons

* Fixes compiler error