Adding docs about the BlackBerry 10 specific widget style

- information how to use it
- which widgets are supported
- the related code is located in QtBase, plugins, styles

Change-Id: I98859c82933be9870faee024f1cc19ff596abd03
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Bernd Weimer <bweimer@blackberry.com>
Reviewed-by: Rafael Roquetto <rafael.roquetto@kdab.com>