Design callback based API

[kanru]

          > I guess for chewing you may want to only show aux/notification when calling the handle_* API, not the set_* that modifies the state directly.

This is a good point.

Callback based API sounds good but we need to think about issues like function reentrant and multi-threading.

For now I'm thinking to stick with current approach and explore callback based design post-1.0 deprecated API clean-up.

Originally posted by @kanru in #506 (comment)