[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'm still not sure what this checkbox actually does. It doesn't seem to do anything on the mcp.php backend.

I would expect that unchecking this checkbox (which is checked by default) would transfer us from viewtopic reports to viewforum reports. But it does nothing as far as I could tell.