Add support for UIPickerViews in Subliminal

UIPickerViews are under a different Window other than keyWindow, so I had to find the other one.  Otherwise it is pretty straightforward.  I'm leaning on the UIA methods a bit more heavily for interacting with the element, but open to suggestions.

Added one fairly comprehensive test case.
Any reason to subclass UITextField and have it be a delegate for the picker? I guess I could envision a specialized `DateTextField` or something that managed its own picker, but in this case it just seemed unusual when compared to using the controller.