Fix floating header vertical shadows glitch

Closes #4340

Fixes a UI glitch where the vertical parts of a box shadow would show.
This fix changes those box shadows for 1 radial gradient, where only 5px of it is shown. No more vertical shadows.

Old:
![screen shot 2014-10-26 at 17 37 03](https://cloud.githubusercontent.com/assets/390392/4783808/b73a4ef0-5d36-11e4-8976-27953da5d787.png)

New: 
![screen shot 2014-10-26 at 17 33 42](https://cloud.githubusercontent.com/assets/390392/4783809/bd485cb0-5d36-11e4-9abd-7e7e83f12b04.png)