[WIP][ticket/12654] Improve header search-box and "quick-links"

http://area51.phpbb.com/phpBB/viewtopic.php?f=108&t=45679
https://tracker.phpbb.com/browse/PHPBB3-12654

![quicklinks](https://cloud.githubusercontent.com/assets/3015760/3172926/a225d112-ebdd-11e3-9307-504afbf071c5.png)

Depends on: https://github.com/phpbb/phpbb/pull/2504

This is a fork of the #2504 branch. I'll rebase this on develop-ascraeus after that one is merged.