docs: def observe_model(self, model):
          """Make this model observable within the controller
  
          The method also keeps track of all observed models, in order to be able to relieve them later on.
  
          :param gtkmvc3.Model model: The model to be observed
          """
          self.__registered_models.add(model)
          return super(ExtendedController, self).observe_model(model)