Add outline when group is groupable

[sourencho]

Remove Circle class and move all functionality to CircleGameObject

• Since CircleRigidBody owned Circle which in turn owned CircleShape, it was very difficult to interact with CircleShape's properties from CircleRIgidBody. I was finding myself writing methods to edit each property. Instead CircleRigidBody now owns a CircleShape and modifies it directly. CircleRigidBody is basically CircleRigidBody + CircleShape.

Add outline when group is groupable

• We do this by using a CircleShape with an outline drawn behind out CircleShape. We couldn't use the original CircleShape because the shader overwrites the outlines pixels.