Rework object locking

This PR reworks the object locking.

It

* Adds note why locking is not required in some cases
* Adds locking where it is missing
* Adds ref counting where it is missing

This is **WIP**, the following modules are on my todo-list:

- [X] `src/objects/`
- [ ] `src/compositor/`
- [ ] `src/action`
- [ ] `src/serialize`

Feel free to propose more, participate in this PR (just notify me which stuff
you want to do) or the like!