More design improvements

I've made some more changes to the design, which might be discussed before merging.

- white top navigation backgrounds
- modified tree icons back to squares (consistency: all other buttons are squares too)
- made all buttons flat (there are no more gradients in the 2.3 design)
- flat search/uberbar button
- added border and box-shadow to the tree (for consistency to other panels styling)
- moved the blue line of active tabs 3px to the top to hide the border of the panel
- modified the user icon to a square with border-radius 3px for consistency (and there are already too much rounded user images in the world)

Left shows the manager before, right side with my changes:
![modx-2 3-manager-comparision](https://f.cloud.github.com/assets/482184/1532641/0afa1c1a-4c7a-11e3-8d9f-f124901f4c15.png)