There might be a chance that an error got thrown after 'contentChanged'
event was fired and stop the `toggleArrow()` method to be called. This
will make sure that the navigation will still be working even some
element in the content is failed.
- responsetype used to be instantiated multiple times(!) in capa_problem
now it is instantiated once, and stored in self.responders
- responsetypes.GenericResponse restructured; each superclass
show now provide setup_response (and not __init__), and may
provide get_max_score(); general __init__ provided to
clean up superclasses.
