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 only did it this way, because it is the use case where we're using it in our app, so I figured I'd validate our scenario was working. Presumably it could be hooked up to any control.

I actually just spent some time playing with the control some more. It can either be hosted in the view itself like any other view element, but it is different when it is the inputView for a textField control. Specifically, it isn't in a different window, which makes sense. This means that the SLPickerView needs to handle both cases however.

I should probably validate that the UIDatePicker variant is works as well.