Cocoa: support QWheelEvent::Phase

The started & ended phases are required for implementing correctly
behaving transient scrollbars (ie. they become and stay visible when
touching the pad with two fingers).

Change-Id: I718d991ba6fd7e949cf9790f3bae285000fce576
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>