Fix a conversion of coordinates of a TouchBegin event

To convert coordinates to a parent's coordinate system one should
translate them by an offset of a child widget relative to it's
parent widget. QRect::moveCenter() doesn't translate, it sets
the coordinates of the touch point's center to this offset.

Change-Id: I9d823784803bd1448c0d665944090674d3ff518b
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>