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.
This was a reasonable assumption but this is actually a bug in `-[SLElement isVisible]` :) Now that we match elements in non-key windows, [this line](https://github.com/inkling/Subliminal/blob/master/Sources/Classes/Internal/NSObject+SLVisibility.m#L279) just needs to be changed to read `self.window` and then the picker will be reported as visible.

Would make a good test case to add to `SLElementVisibilityTest`, too.