* Fix setting root with bottomTabs.visible false
* Fix bottomTabs invisible after hiding through mergeOptions

on iOS, setting root with invisible bottomTabs doesn't work out of the box. We had to change the implementation and load the bottomTabsController children after it has been loaded to the window's rootViewController.

Add iOS support for `bottomTabs.attachMode`.
This property controls when BottomTabs children are attached to hierarchy and when their React views are created.

By default, RNN attaches all children at the same time which might impact loading time as a few screens are instantiated at once.
Using `afterInitialTab` or `onSwitchToTab` generally leads to improved start up time.