call setButtons with empty fab object to remove it

* Implement screenBackgroundColor

* Collapse ViewPagerScreen on TopTab select

TopTabParams are a subset of ScreenParams, moved the properties that
are shared by both classes to BaseScreenParams.
This change was needed to report screen changes properly in ViewPagerScreen

This commit removes necessary hacks which were introduced due to
limitations of previous React Native apis.

Mount/unmount logic isn't coupled to view attached/detached
to window callbacks. Instead, `unmountReactApplication` was added.
We can safely call it when a screen is popped and when application is
reloaded in debug.

* Created Snakbar class to encapsulate Snackbar logic.
* Clicking the action now emits event to current screen
* Action color works