[ticket/12346] Add Template Event overall_header_navlink_append

Adding an event between "BEGIN navlinks" and "END navlinks" allows us to insert custom code in each navlink. This is needed for situations where we want to add extra content, such as drop-down menus to the navlinks.

I decided to use the singular instead of plural name, because it applies to each individual navlink, instead of the navlinks as a whole.

Example use case: http://area51.phpbb.com/phpBB/viewtopic.php?f=81&t=45359

PHPBB3-12346