Core struct, supported
Core accepts user-defined handlers, and invokes them when needed.
Note that only one handler (the first found, according to the
Core::on docs) is called for a given message.
Fn should handle the message/event using the given
Context, and return it back when finished.
Fn should return whether a payload string (you to set the
Buttons) is interesting for a handler to
Events that are supported for event handlers.