[ticket/12336] Add functions_module.php core event

Add functions_module.php core event to allow adjusting parameters
for custom ACP, MCP, UCP modules (location: function list_modules()).

PHPBB3-12336
Merge remote-tracking branch 'rxu/ticket/12336' into develop-ascraeus

* rxu/ticket/12336:
  [ticket/12336] Add functions_module.php core event