Changed Enemy collisions :

- Added a new layer currently called "Destroyable Terrain" but it can be
  enemies as well.
- The previous enemies layer now ignore other object of its own layer.
  This allows enemies to move through one another.
- The new layer is very similar to the old enemy layer, except the object
  on this layer collides with the old and the new enemy layer. Enemies in
  this layer will collide into one another.