[ticket/12339] Add Event core.page_header_after

PHPBB3-12339
[ticket/12339] more complete params and updated _since version

PHPBB3-12339
[ticket/12339] removed return var

need to learn more about events... evidently

PHPBB3-12339
Merge remote-tracking branch 'paybas/ticket/12339' into develop-ascraeus

* paybas/ticket/12339:
  [ticket/12339] removed return var
  [ticket/12339] more complete params and updated _since version
  [ticket/12339] Add Event core.page_header_after