* throwing an error instead of a string

* return a callback to remove the register handler

This commit adds ability to register multiple navigator event listeners:
navigator.addOnNavigatorEvent(func)
navigator.removeOnNavigatorEvent(func)

Event listeners are removed when the screen the navigator is bound to
is unmounted, calling `removeOnNavigatorEvent` isn't mandatory.

As this api conflicts with the current api, addOnNavigatorEvent can't be used
in a screen where setOnNavigatorEvent was used before.

* Make Navigator Public

* Make Screen.js export both Screen and Navigator

* Update Navigation.js to import Screen correctly

* Make Navigator public for Android too

* Forgot to export after importing

* ESLint fix

* ESLint fix

* ESLint Fix

* ESLint Fix

* 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

Change bottom tab bar button image at runtime

Usage is identical to switchToTab, closes #842

* 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