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.

As RNN's Color is used frequently with Android's Color class, we had to use fully qualified names, the rename avoids this.

* fix custom header

* fix e2e

Check LayoutParams before applying, set 0 topMargin if margin is not provided to reset margin.

This reverts commit e1ee3d5f99.

This reverts commit f6728fc1d6.

This reverts commit 2d879ccd0c.

This commit fixes a crash when hardware back button was pressed before root was set.

The diff does not render properly on https://wix.github.io/react-native-navigation/v2/#/docs/Installing?id=you-can-use-react-native-navigation-o. I believe the proposed change should fix this.

This prevents the back button from being added too early when pushing screens.
If the back button is added before the actual push (current behaviour) the the previous screen's
title moves unexpectedly to make space for the back button.

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.