A action must implement following 4 functions.
- start,
- step,
- stop,
- free,
- *_new(), and
- add *_new() to animate::h .
*_new() must invokes
mb_word_add_action() to add new object as one of actions in the word specified as an argument of it. It also means *_new() must have an argument with type of (mb_word_t *).