QScrollBar: handle QWheelEvent::Phase for transient scrollbars

Transient scrollbars should become and stay visible when touching the
pad with two fingers.

Change-Id: Ic36ba56019f484cedb2404550551ebe68ea6f285
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>