Re: [uievents] Expose 'direction-inversion' in wheel events (#57)

FWIW, the precedent for when the value can be unavailable is nullable attributes, like on [`DeviceOrientationEvent `](http://w3c.github.io/deviceorientation/spec-source-orientation.html#deviceorientation) and friends. So `true`, `false`, `null`, in that case.

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