[ticket/12831] Optn to prevent setting HTTP headers

https://tracker.phpbb.com/browse/PHPBB3-12831

PHPBB3-12831
[ticket/12831] reimplemented headers as array

https://tracker.phpbb.com/browse/PHPBB3-12831

PHPBB3-12831
[ticket/12831] simplified code, upd event block

https://tracker.phpbb.com/browse/PHPBB3-12831

PHPBB3-12831
[ticket/12831] fix CS

https://tracker.phpbb.com/browse/PHPBB3-12831

PHPBB3-12831
Merge pull request #2730 from omniError/ticket/12831

[ticket/12831] Allow events in page_header functions to prevent the setting of HTTP headers