qmlRegisterCustomExtendedType, qmlRegisterExtendedUncreatableType

There are cases in Qt3D where we'd like to be able to register uncreatable
extended types. The main use case is having an abstract class that has an
extension class so as to have a clean separation between the C++
and QML API. Implementations of the abstract can then be easily registered
to QML and rely on the extension class for QML specific properties.

The other feature we'll need in the near future is the ability to create
extended QML types that use a custom parser.

Two new type registration method were added to qqml.h to fulfill those
needs. Unit tests for those are present in qqmlecmascript and qqmllanguage.

Change-Id: I15b2cd791ffd36b537305af1873491c079d4094e
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>