Entity Layers + physics collision matrix

[schteppe]

• Added a .layer property to Entity instances.
• Added LayerManager that keeps all layer names. Can be used to convert between name and layer. Good to have when this stuff gets into Create.
• PhysicsSystem has a "collision matrix" with information about what layer collides with what.
• Colliders update their masks on initialize() depending on the "collision matrix" in the PhysicsSystem.