Doc: update the Qt Quick app tutorial

The wizard in 3.1 uses a Window QML type instead of a Rectangle,
which means that states must be placed within a statusGroup.
The example can no longer be built from Qt Quick 1 elements.
Replaced \snippet commands with \quotefromfile commands.

Change-Id: Ia2cec2b8b638913a2b9b2e27b36e6f2f1ffc4a49
Reviewed-by: Alessandro Portale <alessandro.portale@digia.com>
Reviewed-by: Marco Bubke <marco.bubke@digia.com>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>