Fixed DropShadow issue in NavigatorWindowStyle

Fixed an issue with the DropShadowEffect in the NavigatorWindowStyle of
AvalonDock.

***Before***
![sd_before_window](https://cloud.githubusercontent.com/assets/1153480/4295327/d5625f2a-3de8-11e4-8d42-d66b2e1badf5.png)



***After***
![After](https://cloud.githubusercontent.com/assets/1153480/4295324/c5128b72-3de8-11e4-976b-1a8f39e33f99.png)