QtProtobuf: Converter/(de)serializer registration

With types being registered mostly automatically all that's left is to find a way to register the converters and (de)serializers.

Since this may be the first interaction a user has with a type it's conceivable that the type has not yet been initialized and thus has not had a chance to be registered. So, we need to look into ways this could be done. The types are not (at least currently) in plugins so we cannot just load and run a function to achieve this.