Doc: Use proper module name for Qt WebEngine Widgets

Fix the name used for the \module command to include the
module name to properly generate the C++ class reference
page for Qt WebEngine Widgets, and link to the new page
from the module landing page.

Change-Id: If0814650fa16ecf1b5c66e8e46e598d576a8c21d
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>