This commit started with a small fix to a TopBar visibility issue detailed in #3971 which unveiled
a few issues with how options are resolved and merged.

TopBar visibility
The initial TopBar visibility issue was a result of hide animators now being cancelled when pushing another screen.
Another issue was that if the TopBar was not measured (Initial stack child was shown with `TopBar.visible = false`),
the start value passed to the Y translation was 0. This commit makes sure the TopBar show animation starts after the TopBar is measured.

Options issues
When resolving current issues in ParentControllers, the options of the current child were wrongfully taken into account.
This commit changes options resolve logic so that initial options (and options dynamically merged to them via mergeOptions call) are merged.

Unmount buttons when corresponding screen is unmounted

Currently buttons were unmounted each time a screen was pushed to the stack.
As we're clearing props when screens are unmounted from the prop store - when a screen
was popped to - its buttons's props were undefined.

This commit changes things so that buttons are unmounted when buttons are set
using mergeOptions and when the corresponding screen is unmounted.

Unmount Title component only when related child component is destroyed

Currently TopBar title component, background and buttons are unmounted when a new screen is pushed to the stack.
This causes a bug when props are passed to these components as props are cleared when child components unmount.

This commit fixes this bug for title components.

* Refactored bottomTab and bottomTabs

* Initial commit before switching to our fork of AHBottomNavigation

* Bump AHBottomNavigation version

* More work on BottomTab options

Need to merge some child options when a child is attached to parent

* Fix tests

* DefaultOptions refactor

* defaultOptions are now merged before applying options
* Handle BottomTab options in dedicated presenter
* Create BottomTabs in BottomTabsController.createView() instead of constructor

* Set bottomTab TypeFace

* Update Android installation guide

* fixed unit tests

* Fix unit tests

Kind of a workaround

* Handle NPE in getPreferredHeight

If a view hasn't been attached yet, return 0 height

* Apply BottomTab text and icon color individually

wix-playground/AHBottomNavigation does not support setting color for the entire tab (icon and text),
instead it supports setting color individually to text and icon. Another change is that it no longer supports
default color for icons. If icon color is undefined - it won't tint the icon.

* Add getCurrentChild to ParentController contract

* Clear backButton when setting stack root

* Add  ViewController.resolveCurrentOptions

resolveCurrentOptions merges the controllers options with all of its current children

* StackController refactor

* Create pushed child view only in StackController.createView
* When initialising a stack with multiple children, only create the top child's view
* StackController constructor now accepts children
* When pushing child to stack, apply options that change LayoutParams

* Apply BottomTabs layout options when tab views are created

* bototmTab changes in iOS

* options fix

* Fixes options propagating

* Refactored backButton api

* Move StackController to separate package

Implement BackButtonOptions

* Somewhat simplify StackController creation in tests

* Pull backButton logic from StackController

* Handle all TopBar button options in TopBarButtons

* fix back button on iOS