Backport new composer toolbar icons to 0.9.x

This removes a bunch of crufty icons that haven't been used in years, and also replaces the patchwork toolbar icons for Composer with a matching set of flat icons adapted from the Font Awesome project.

This is a backport of #922 to `0.9.x`.

![screen shot 2014-09-01 at 4 20 00 pm](https://cloud.githubusercontent.com/assets/494338/4111868/66bedf1e-3215-11e4-893e-e2828de00ca1.png)