Move Qt Quick Widgets example and document it

Because QtQuickWidgets is part of Qt Quick module documentation,
its examples need to also be moved under the examples/quick
directory structure.

This change moves the example, creates minimal documentation
for it, and adds a link to it from the QQuickWidget class
reference.

Task-number: QTBUG-37272
Change-Id: Iffb67849f150b9aaf0edaef5852364e93f7752b8
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>