Fix for tapping alert view buttons on iPads

[percysnoodle]

On iPads, alert views are inside a _UIAlertNormalizingOverlayWindow whose coordinate space is not the same as that of elements' accessibilityFrames. This commit converts the accessibility frame into the main window's coordinate space instead, allowing alert view buttons to be tapped on iPads.

Also fixed is a bug whereby a tap step would seem to succeed if tappablePointInRect: returns (NAN, NAN), which is what currently happens on iPads, even though the element is never tapped.

[percysnoodle]

I've updated this to work with the latest master.

[bnickel]

I have not seen alert views fail on iPads with the KIF 2.0 test suite. Could you provide a failing project or failing tests?

[bnickel]

This issue was resolved with a different (but similar) approach from issue #60. This approach uses the current window's coordinate space unless is has a transform applied to it. If it does, we then search the available windows for the first one with no transform applied to it.