[ticket/13059] Add core event to generate_page_link()

Add core event to generate_page_link() to allow overriding/modifying pagination URLs.
Event request: http://area51.phpbb.com/phpBB/viewtopic.php?f=111&t=45905

<a href="http://area51.phpbb.com/phpBB/viewtopic.php?f=111&t=45905">PHPBB3-13059</a>.
Missing the block:
```php
 /** @var \phpbb\event\dispatcher_interface */
protected $phpbb_dispatcher;
```

It's also missing for the controller helper. Can you add that aswell?