Change context menu nav icons (#279)

Fixes #279 
The icons are now 20px wide, here is a comparison of vanilla and my changes:
![vanilla](https://cloud.githubusercontent.com/assets/107189/5316068/4617d0b0-7c87-11e4-86b8-2d33bea0a02a.png)
![gnome](https://cloud.githubusercontent.com/assets/107189/5316069/475eb664-7c87-11e4-95b5-86503ae538be.png)