Allow use of template class instances inheriting from a Q_GADGET in Qml

The Q_GADGET macro cannot be used in templates. It can however be useful
to derive a template class from a Q_GADGET enabled base class to benefit
from type safety features in C++ (e.g. the class could represent an id
or handle for some C++ type).

For proper wrapping of a QVariant with a gadget value in a QJSValue, the
QMetaType::IsGadget flag must be set for the registered template
instance type - which does not happen prior to the fix because
IsGadgetHelper requires qt_check_for_QGADGET_macro to be defined in the
registered class but not in an ancestor class - in other words: The
class must declare Q_GADGET.

To overcome this, IsGadgetHelper/IsPointerToGadgetHelper can now
differentiate between a Q_GADGET flagged class (allowing
automatic registration) and a derived class, e.g. a template class
(forcing Q_DECLARE_METATYPE to be used explicitly).

[ChangeLog][QtCore][QMetaObject] It is now possible to use template
class instances inheriting from a Q_GADGET in Qml

Task-number: QTBUG-66744
Change-Id: I7632ad45cff79fa422b3f852ca0b963f35fab155
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>