Add QWheelEvent::source

This is analogous to 0a92295c


which added QMouseEvent::source.

For now, we say that a wheel event is synthetic when it comes from
a trackpad or other device that provides scrolling by some means
other than an actual wheel.

Change-Id: I0452ca2080b551b18b9c2f6e42db925d14ae339e
Reviewed-by: Morten Johan Sørvig <morten.sorvig@digia.com>