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.
It's nice to retain UIAutomation concepts where possible, i.e. your reference to "wheels", but let's avoid going so far as to provide equivalent UIAutomation code--that strikes me as too much information.