Doc: edit qtwinextras module documentation

Added a lot of articles, fixed typos and grammar errors,
enforced the Qt doc writing style, and used standard wording
for qdoc commands.

Change-Id: Ia644907e5d8454ddd789ab214a93fe7524aaf206
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Geir Vattekar <geir.vattekar@digia.com>
Reviewed-by: Jake Petroules <jake.petroules@petroules.com>