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.
I've temporarily worked around the issue by writing a custom isVisible method that relies on the UIA implementation for this element.  It might not be perfect, but it is good enough to get me unblocked for now.