We should also support showingModals on Android regardless of root

Some extra help concerning a common issue that occurs with fresh install of Xcode + react native 0.56. https://github.com/react-community/create-react-native-app/issues/316#issuecomment-351371080

When fresh installing `v2` branch some time getting issues with bundling. Due to `rn-packager` cache. Normal `npm start -- --reset-cache` was not working properly. So had to reset the cache manually. Maybe its because of the start script is different from regular react native app setup

Fix an imprecise path name, and fix a meaningful typo `No Run --> Now run`.

Change `index.ios` to `index` in `AppDelegate.m`.

Fixing 404 not found

Apparently, UiImplementation doesn't like it when views disappear from hierarchy unexpectedly.
As yellow boxes are added to all ReactRootViews, we remove them from TopBar components.
When these components got unmounted, UiImplementation tried to remove the yellow box and since it
couldn't find it - it crashed.

This commit simply adds the yellow box before views are unmounted.

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.