[ticket/12782] Use an interface for the phpbb event_dispatcher

PHPBB3-12782
[ticket/12782] Add doc block

PHPBB3-12782
Merge pull request #2672 from Nicofuma/ticket/12782

[ticket/12782] Use an interface for the phpbb event_dispatcher

* Nicofuma/ticket/12782:
  [ticket/12782] Add doc block
  [ticket/12782] Use an interface for the phpbb event_dispatcher