[ticket/12942] Add core.add_form_key core event

Add core.add_form_key core event to allow extensions modifying
form key.

PHPBB3-12942
Merge pull request #2845 from rxu/ticket/12942

[ticket/12942] Add core.add_form_key core event

* rxu/ticket/12942:
  [ticket/12942] Add core.add_form_key core event