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* rm outdated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