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 is a clever way to avoid searching multiple windows but I think it might be alright to just do that. We probably don't have to search through all the windows, just the key window and those above it:

```diff
-            accessibilityPath = [[self accessibilityPathSearchRootElement] slAccessibilityPathToElement:self];
+            NSArray *windows = [[UIApplication sharedApplication] windows];
+            UIWindow *keyWindow = [[UIApplication sharedApplication] keyWindow];
+            NSUInteger keyWindowIndex = [windows indexOfObject:keyWindow];
+            NSAssert(keyWindowIndex != NSNotFound,
+                     @"`[[UIApplication sharedApplication] windows]` should contain the key window.");
+            
+            for (NSUInteger windowIndex = keyWindowIndex; windowIndex < [windows count]; windowIndex++) {
+                UIWindow *window = windows[windowIndex];
+                accessibilityPath = [window slAccessibilityPathToElement:self];
+                if (accessibilityPath) break;
+            }
```

And then elements wouldn't require any special configuration depending on where they displayed.