[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

PHPBB3-13059
Merge pull request #2947 from rxu/ticket/13059

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