Add basic find previous/next feature

I've added two buttons next to the search to be able to find Previous and Next.

Nits:
- `Shift`+`Enter` invokes a "Previous" Search.
- Closing search clears also the selection (like Chrome does)

![screenshot from 2014-01-27 11 48 27](https://f.cloud.github.com/assets/634478/2007999/af93dc60-8740-11e3-9e83-5fadac06828a.png)