Stack related options are applied only if the view is visible, for now this is a reasonable fix.

This needs to be addressed for other ParentControllers as well

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.