Taskbar button height fix in vertical panel layout

When razor-panel is vertical every taskbar button is expanded verticaly. Here is the small fix.
Before:
![before](https://f.cloud.github.com/assets/101042/212743/31e2d9b0-832c-11e2-8bce-f9f768174eed.png)
After:
![after](https://f.cloud.github.com/assets/101042/212744/37b60f24-832c-11e2-8188-b1e97772a3d1.png)