introduce VSWindowStyleKey for #904

also fix GlowWindowBehaviour to use styled window properties
e.g. a GlowBrush can now set via style

![2014-03-10_13h38_15](https://f.cloud.github.com/assets/658431/2373485/f3d08df2-a850-11e3-8d1c-330df122e1de.png)

Closes #904