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.
Also create the explorer view controller lazily.
Eliminate ambiguity when using the property, ivar, and method boxes.

Some users have reported build errors in these areas. They likely have conflicting methods that get imported through a PCH. For example, if the compiler sees a definition of another method called “property” that returns an NSString, it doesn’t know which one to pick.
Merge pull request #14 from JaviSoto/feature/lazy-window-creation

Lazily creating the FLEXWindow when it's actually needed.