Doc: Define Qt global object as a QML type and fix referring docs

This change adds the Qt global object as a QML type in the Qt QML module
and fixes links to it in documentation.

Task-number: QTBUG-29353

Change-Id: I1690224091a79a3309fac7a78aa0c0c734449e4f
Reviewed-by: Mitch Curtis <mitch.curtis@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>