Use QVideoWidget in the videowidget example.

The videowidget example now uses QVideoWidget instead of a custom
QAbstractVideoSurface, the former being the most straightforward and
fastest way of showing a video in a QWidget.

The old code has been moved to a new example: "customvideosurface".

Change-Id: I15fb4482d0e77bafd24d4cb03316fc8f4ef13c35
Reviewed-by: Lars Knoll <lars.knoll@digia.com>
Reviewed-by: Jason Barron <jason.barron@digia.com>