Add texture pack selection.

Almost done, still needed:
- A screenshot for default texture pack
- Fix the textures-still-in-cache problem (i.e. : start the game, exit to menu, change texture pack, restart, textures have not changed).

What are your opinions on it?
![tpscreenshot](https://f.cloud.github.com/assets/2923554/899181/78f8c698-fb36-11e2-8e6c-37db82f77cd0.png)