Organize project structure

Grouped files in to separate packages:

- `gl` contains classes such as Rectangle, Point, Color etc. (GL utility
  classes).
  - `glui` is now `ui`, containing just GLUI components.
- `game` contains all of the game-related classes, e.g. Player, World.
  - `game` has a subpackage called `world` containing world-related
    classes.
- General-use utility classes are now in the `util` package.