Refactoring Motion2D

[tigleym]

Entities whose Motion2D components with a rigid_body property set to true should be able to neglect any forces exerted on it. So if entity 'A' collides with an entity 'B' with rigid_body, entity B's Motion2D component will not be affected by the collision.

[tigleym]

Update:

We'll break this issue down into smaller tasks since the goal is simplifying a lot of the existing motion2D logic and will require rewriting some systems responsible for handling entity motions. PRs related to the motion 2D refactoring should reference this issue.

[cdsupina]

#117