Improved UI for the PDE header

I felt that the editor UI could use some attention. No dramatic shifts here – I just aligned some elements, unified the fonts and made the tab bar a little clearer.

![prettyprocessing](https://f.cloud.github.com/assets/514315/567067/4247cf94-c6a8-11e2-884a-897312091aff.png)

I hope this is useful!