Fix rendering bug when hovering change

![screen shot 2014-09-22 at 08 12 33](https://cloud.githubusercontent.com/assets/57446/4352181/8124a7fc-421f-11e4-8351-91fec04d6851.PNG)

There is a bug in Xcode 6 under OS X 10.10 when you hover a change in the sidebar.
This is fixed by removing .wantsLayer on the popover.
I also removed the corner radius as it no longer makes sense for it to be there.

This is how it looks like without the .wantsLayer property set to YES.

![screen shot 2014-09-22 at 08 14 30](https://cloud.githubusercontent.com/assets/57446/4352191/c534146e-421f-11e4-9fdb-a640309c6e46.PNG)