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.
Methods that talk to UIAutomation (using the `-waitUntilTappable:...` methods) need to document some standard exceptions that may be thrown in the process. They always need to document that `SLUIAElementInvalidException` may be thrown. If they wait until tappable, they need to document that `SLUIAElementNotTappableException` may be thrown too. (This would apply to `-selectValue:forComponent:` except for #214.) You can copy the boilerplate for this from `-[SLTextField setText:withKeyboard:]`.