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.
Oh yeah, falling back to `UIAElement.isVisible` is a great temporary fix.