Add icons on the tracks list, change his layout a bit.

Second step for issue #33.
The layout chosen in PR #35 seems a bit weird when there is no icon on a track, isn't it?
![tracks list with icons](https://cloud.githubusercontent.com/assets/833234/4852181/0df7421c-6079-11e4-9f9d-8376b674d79a.png)