Unmount react views when bundle load completes

Users providing their own ReactNativeHost implementations should implement BundleDownloadListenerProvider
and make sure they set DevBundleDownloadListener when ReactInstanceManager.

* V2: Peek and pop

* Add event when preview is dismissed in non commit mode

* tslint

* Refactor playground. Document preview api

* Added width prop

* Implement hybrid push/preview button

* Fixing small linting errors

* Refactor TopBar options, introduce title options

* Update background to new api

* Refactored topBar options on iOS

* Initial react component in TitleBar android

* set height to react view in TitleBar

* fix tests and lint

* ios unit tests fix

* 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

* default options support
* refactored navigation options

* refactored e2e tests to work with testIDs

* lint fix

* android e2e fixed

* push/pop promise

* tests

* modal

* other

* fix tests

* probably test fix

* changed this.props.id to this.props.containerId

* fixed js tests

* refactor