Doc: created an overview topic for Qt Windows Extras

Pulled introductory information from the code source files
to a new file called qtwinextras-overview.qdoc.

Change-Id: I93247a99638ecb8d98c58859b1bf2333963d2848
Reviewed-by: Ivan Vizir <define-true-false@yandex.com>
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Reviewed-by: Jake Petroules <jake.petroules@petroules.com>
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>