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.
Not sure I follow the "no way to specific accessibility information", but I have added "To manipulate a date picker control, use SLDatePicker as it isn't a direct subclass
 of UIPickerView."  Is that sufficient?