Remove components at runtime

> ~~This issue is currently blocked by #27.~~

Allow the editor to request that an existing component be removed from an entity at runtime.

To do this, add a variant to `IncomingMessage` called `RemoveComponent` that specifies the ID of the component type (i.e. the user-provided name of the component) and the entity from which the component should be removed. It probably makes the most sense to add the logic for actually removing the component to `WriteComponentSystem`, since it already requests write access to the appropriate component storage.

I'm happy to answer any questions or provide mentorship for anyone who wants to take on this task 🙂