Lazily creating the FLEXWindow when it's actually needed.

This allows users of FLEXManager to use the -registerGlobalEntryWithName:objectFutureBlock: API in a +load method of their app. Right now that creates the UIWindow object when UIKit is not ready. With this change, the window will be created the first time you call -showExplorer, which would also help a bit with launch times in apps that have FLEX because creating that object would be delayed from app launch to the first time it's used.