[WIP][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.
Not sure why this one has an id. I searched through all the CSS and JS files and can't find any instance where it's actually referred to.