Doc: using UI forms in Qt Quick Designer

New concept in Qt Creator 3.3 and  Qt 5.4.

Change-Id: I24cacd4c98bb6873625d5cda44a78d5ba0ad8f5b
Reviewed-by: Tim Jenssen <tim.jenssen@digia.com>
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>