Tweaking of mod selection window

- Draw scrollbars on the borders and use whole height of list windows.
- More space for description.

Before:
![before](https://f.cloud.github.com/assets/5421954/1998375/9967518e-8532-11e3-9b99-ab4dd39a205e.png)

After:
![after](https://f.cloud.github.com/assets/5421954/1998376/9ae28e0c-8532-11e3-82e3-a2c5e8183d93.png)

P.S. This PR looks even better with my recent scrollbar fix.