Let QMetaType know which types are Q_GADGET

This is required so we can take a QVariant and detect that it contains a
Q_GADGET and then use method like QMetaType::metaObject and QMetaProperty::write
with the QVariant::data

Change-Id: I3603692e4e84426e10bf59949e3def3ea4947bec
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>