Add support for using textures in hotbar

Testing images:
![hotbar_selected](https://f.cloud.github.com/assets/3742497/893730/e1fc8fde-faab-11e2-813c-73003a93d98f.png)
![hotbar](https://f.cloud.github.com/assets/3742497/893731/e69b06f6-faab-11e2-8b1c-d7ace246701e.png)