move toc-node to common

Depends on commits in #90 and probably #53.

ToC nodes are used in `gh-book` and `atc` but behave slightly differently (dropping in `atc` results in a link but dropping in `gh-book` results in a link or a copy).

The "slightly different" logic is defined in the subclasses (inside `addChild`)

# Testing

- [ ] create a chapter
- [ ] move chapter around inside the ToC
- [ ] change title of a piece of content