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

https://tracker.phpbb.com/browse/PHPBB3-12831
Okay so we just had a nice idea about this.
Please make the headers an array 
```php
$headers = array(
	'Content-type' => 'text/html; charset=UTF-8',
	'Cache-Control' => 'private, no-cache="set-cookie"',
	'Expires' => 0,
	'Pragma' => 'no-cache',
);
if (!empty($user->data['is_bot']))
{
	// Let reverse proxies know we detected a bot.
	$headers['X-PHPBB-IS-BOT'] = 'yes';
}
```
and pass them to the event. Then extensions can add and remove headers as they want.