Made it so removing actors from the scene unbinds the OnDisposed functions.

- This also included having the Scene2d Dispose unbind all OnDisposed events from its actors.

#44
Added test to verify that Scene2d remove and dispose unbinds actors OnDisposed.

#44
Made the CollisionManager disposable.

- On dispose the CollisionManager unmonitors all of its collidables and unbinds all of their OnDisposed events.

#45
Added test to verify that CollisionManager's Dispose works as intended.

#45