Change Find Usages to use the Quick Panel and preview the file too

The existing Find Usages buffer was worthless - you couldn't navigate to where it actually was. I changed it to use the Quick Panel, with a preview of the file, just like Sublime's native Go to Symbol. Awesome! Now it's really usable.

![image](https://cloud.githubusercontent.com/assets/1989335/4538803/40b814a6-4df5-11e4-88ac-42844aaf2fac.png)