Add mouse wheel events handler to MouseArea

This patch was based on the attached patch in QTBUG-7369. It basically
exposes the wheel events to MouseArea via the onWheel signal.

The current API is based on the new QWheelEvent API introduced by this
patch: http://codereview.qt-project.org/#change,12532



Task-number: QTBUG-7369
Change-Id: Id58513715c2d0ae81e3a69e9e1ed400bbae07507
Reviewed-by: Michael Brasser <michael.brasser@nokia.com>