Foundational concept design: version control

**RFC link:** [Version Control](https://github.com/ForNeVeR/praefectus/blob/HEAD/docs/rfcs/version-control.md)

Praefectus should implement a simple version control system ([VCS](https://en.wikipedia.org/wiki/Version_control)) for the information it contains. Every task should have a fully persisted change history with information about the nodes that were involved in changes.

The history should be a DAG (the same as in Git), since multiple tasks created within different "branches" could then be merged by the user into one entity.

Should every node maintain its own branch? How should import/export process cooperate with that? We'll need to solve these questions in this task.

RFC is ready, so this may be closed after finishing the initial implementation:
- [ ] #32
- [ ] #33
- [ ] #34
- [ ] #39