[3.2][ticket/12649] Change sort & display options in footers to dropdown menu

https://tracker.phpbb.com/browse/PHPBB3-12649

![display-options](https://cloud.githubusercontent.com/assets/3015760/3282015/af48f930-f4d1-11e3-8ab2-0e644562cf6d.png)

Somewhat depends on https://github.com/phpbb/phpbb/pull/2504 for icons.
I wanted the "big button" icons and "post icons" to remain separated, but I can change it if needed.