* Update part of usage.md to reflect new API changes

* Update usage.md

It was handled inappropriately and some touch events were
intercepted unintentionally.

* adresses #1078

Continued work on Android

* Rename TopTabsLayout to TopTabsViewPager
* Pass ScrollEventListener from component
* Change ViewController.view to protected
* Don't keep ref to view in ComponentController
* Pull IReactView and ReactViewCreator to separate classes
* Ensure ToolBar.menu is created before clearing menu items
* Unmount react view before destroy
* Let TopTabsController set mediator in view visibility handling

* Support creating stack with multiple children

* Add e2e

* Initial overlay implementation on Android

* Move logic to touch delegate

* Implement dismiss Overlay

* Overlay e2e

* fix iOS e2e

Currently the default NavigationReactNativeHost only adds the NavigationPackage. Users of React Native Navigation may need to override additional methods in ReactNativeHost. This change allows advanced users to provide their own implemenation of ReactNativeHost, with the requirement that they add the NavigationPackage themselves.

Closes #653

* overlay support

Only reason to offload icon loading to background thread is to avoid
supposed network on main thread calls which happen only in debug since
icons are passed loaded from localhost.

* Change topBar backgroundColor to Color object

* StackLayout implementation

Unfortunately, this commit also contains a lot of refactoring and a few
bug fixes relating to styles.

* TopBar is not part of Component, instead it's a part of StackLayout
  and shared between all components pushed to the stack Similar to iOS.
* Handle ScrollEvent in ReactView instead of passing the listener down
* Implement Fraction and Text null objects for style params
* Converted a few style params to null objects. This fixes some styles
  being applied even when they were not specified explicitly.
* Stop sending null promise to commands
* Seperate push and pop into animatePush/push and animatePop/pop
  to avoid boolean param

* Adapt TopTabs to new API