Doc: Add a module page for Qt Quick Widgets

Although the documentation is part of Qt Quick, a separate
module page with a \qtvariable command is still needed to
have correct information about which module to link against.

Task-number: QTBUG-37272
Change-Id: Ie00e9418be4feec299cb36c526ca366504c89008
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>