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.
Definitely. Though you just reminded me, I think it would be nicest to have tests of `SLPickerView` and `SLDatePicker` where we just test them as regular views.

The case where the picker is an input view exercises the picker view itself less than Subliminal's ability to match elements in other windows (especially if we make [this change](https://github.com/inkling/Subliminal/pull/211/files#r13839036))--I think the input view scenario could be an `SLElementMatchingTest` case, actually.