Capability to register listeners (observers) on read and write #39

We need to clarify

• the callback signature: probably just (obj, field) for get and (obj, field, value, has_changed) for set.
• should the observer be called in the same thread ? probably yes for now. Later we could imagine using asyncio.loop.call_soon but that seems overkill for now
• should the observer be called after setting the value in case of a set observer ? Maybe better to support (A) generators with a single yield and (B) normal methods, in which case they will be called after* the field is set.
• the name of the option in field() and in the associated decorator (like for validators and converters). Maybe get_observers and set_observers (without the 's' for decorator and corresponding add_xxx method) ?

See https://traitlets.readthedocs.io/en/stable/using_traitlets.html#observe and https://python-3-patterns-idioms-test.readthedocs.io/en/latest/Observer.html