API leakage with the `on-open` action

[cantsin]

With the new on-open action, we inadvertently leak some internals to outside, specifically our openNodes list which contains Node classes. I'm not yet sure how to address this.

[taras]

It might also be ok! Especially, if it's possible for a Node to be extended. I'm not 100% on this. I can ask around.

[taras]

Here is an example of using ES6 classes to provide an interface https://github.com/flexyford/impagination