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.
Actually a proper fix is slightly more complicated than that: https://github.com/inkling/Subliminal/issues/212. Can you leave the assertion as is, but add a "to fix" comment with a link to #212?