Update contact auto-completion on contact creation

WIP.
This is a proof-of-concept contact list update on contact creation.

To-do:
- [x] Also update list on contact deletion (should be easy)

Possible future to-dos (to be discussed):
- [x] Should we allow multiple observers if we don't need them right now?
- [ ] Should we pass all the contacts when only one was created?
- [ ] Should we provide fine-grained events notification (e.g. `onDelete`, `onSave`...) ? Then `onChange` would just call them all...