Stack notification buttons vertically so they're readable

So instead of this:

![buttons are too small and their text is replaced by dots](https://f.cloud.github.com/assets/262776/943388/2dc309fc-0213-11e3-902a-7601e614c831.png)



We get this:

![diskspace](https://f.cloud.github.com/assets/262776/943387/0a5c7aac-0213-11e3-87f8-34f73c05f707.png)