Non-local enums should be accepted as QML signal and method parameters

Currently QML only supports enums as signal and method parameters if they are declared in the same class as the signal or method. This is a limitation inherited from QMetaObject, that doesn't allow QML to determine if the parameter type is an enum, or just some other random type.

The ideal solution would be to extend Q_ENUMS() to support "registering" an enum from another class. For example:

class Foo : public QObject {
    Q_ENUMS(Bar::CustomEnum);

slots:
    void myMethod(Bar::CustomEnum);
};

in this case, all QML really needs is for the Bar::staticMetaObject to appear in the Foo::staticMetaObject's extra data list.