Make collision / physics system moduable

[nhurde]

Similar to the artist/canvas and sound classes, the collision system should be modifiable via interface/inheritance to swap between implementations depending on what is created.

Useful in the event of switching to Box2D, PhysicX, etc.

The default method is currently spatial portioning, so this would also become a module called GPE_Spatial_Collisions.

Collisions are supposed to be inter and intra Layer depending on project settings.