QVariantMap <--> QVariantHash converters

QVariants internal converters do not allow to convert a QVariantMap to a QVariantHash and vice versa. That alone would not be a problem, but QMetaType::registerConverter does not allow to register a converter for built in types (for whatever reason), making it impossible to create those.

One example of a usecase would be: I store a QVariantHash as a json object. When I try to read it back later, it fails, because QJsonObjects are converted to QVariantMaps, and they fail to convert to a hash.

It would be nice to add those, or make it possible to register converters for built in types.