* remove default values

* fix test

* options

* proof of concept

* pop

* test fix

* default options

* scale

* rotation

* null check

* refactor

* refactor

* refactoring

* fix tests

* 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

* Implement titleFontFamily in topTab option

* docs

* Pass TypefaceLoader instead of context

Slightly changes topTabs node declaration and removed the container from it.