Add "Styles" example

Based on the "Styles" tab of the Gallery. The motivation for this
change is to cleanup the Gallery example so that it eventually runs
nicely on various desktop and mobile platforms without major
modifications.

Task-number: QTBUG-41307
Change-Id: Iffcd3557bf7c77f5a10bda8bfda5ddb636dd5ac1
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>