Previously we weren't triggering transaction finish when going from none active screens to 1 active screen. This turns out to be the case in tab navigator where for a sub-frame moment the active state changes for the current screen to `NO` and then new screen isn't active yet.
Fixes#53
This is a follow up to #48 which makes Screen component ignore setting of pointerEvents. As it turns out react-navigation tries to set this setting and in addition to handling it manually we also need to prevent it to be set via React prop.
This change automates first responder restoring when screen is deactivated and the activated back (e.g. when we push new screen on top and then go back). In addition we disable pointerEvent setting for the Screen component as changes made to that prop would cause underlying views to resign responder before we can remember it. Because of that we add an automatic handling for pointer events for the Screen component that disables all touch interactions when screen is transitioning.
With this change in the screens that are being transitioned get detach+attach events similarily to how it is done in UINavigationController. This allows views underneath to get notified that the transition is started and react to it when necessary (e.g. hide keyboard)
