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

* default options support
* refactored navigation options

Application class can now be accessed statically using Context to access AppModule
which provides dependencies needed throughout the app.

* Deleted getContentView method
* Delete Custom dialog code
* Delete topTab code
* sendOnNavigationButtonPressed was pulled to a dedicated interface

* Ensure ReactView isn't created after it's destroyed

Fixes #2490

* Rebase fixes and renames

https://github.com/wix/react-native-navigation/issues/1397

* readme

* doc

* topbar fix

* rm doc

* options presenter fix

* rm userless stuff

* WIP

* WIP

* WIP

* translation

* WIP

* todo

* draw under

* fix merge

* merge fix

* more merge fix

* improve calc

* animation improvement

* fix tests

* refactor

* refctor

* tests

* fix test

* anim improve

* rm outdated

* refactoring

* more refactoring

* tests

* refactor

* hotfix

* more refactoring

* refactor

* some more refactoring

* refactor

* Collapse topbar

closes #2487