Create a new component called "Velocity" to generalize things that move
in a specific velocity every frame, and create a system that applies
that velocity to an entity's position in an isolated, testable, and
framerate-independant way.

Also remove the speed/velocity calculations from the UserProjectile
component so we can use the new Velocity component instead.

Give entities a way to communicate with each other via the
MessageDispatchService. The `MessageInbox` component is used to receive
messages. Currently the MessageDispatchService is for entity-to-entity
communication. Global events will be handled by another service,
although will probably use the same `Message` object with `EventArg`
parameters.