Event System

Requirements: Events should be emitted by the package

This should be done on every update/delete/delete_all handled by the HasPreferences Trait -> 3 Events

An event  includes:
- PreferenceGroup (only update/delete)
- PreferencableModel
- Authenticable
- maybe:
  - mixed old_value
  - mixed new_value(only update, for delete/delete_all its null)