Improve size, positioning, and color of category icons in desktop dropdown (bug 1055701).

r? @ngokevin @spasovski

This messes up the positioning of icons in the categories page, but that will get changed as part of the overhaul of bug 1055313, so I'm going to leave it for now.

![catmenu](https://cloud.githubusercontent.com/assets/23885/3971340/a1a91c90-27d2-11e4-9a78-378ff357da75.gif)