Collision handling

[SiXoS]

A lot of games will need collision handling. It would be nice if there were some utilities for collisions.

There are basically four iterations of this that should be implemented iteratively:

1. Invoke some function that checks if two shapes intersects.
2. Return the collision normal for an intersection
3. Have some listener, similar to the input block that is invoked when two objects intersect. This should require the game creator to mark which shapes that can collide and which shapes that can collide with which other shapes. This is to reduce the number of checks required.
4. Physics. By specifying some parameters (like gravity, weight of shapes, elasticity of shapes etc.) to shapes they should act as objects in a physical space (bouncing off of surfaces, falling down from gravity etc.).