Create and destroy entities at runtime

Add support for allowing the editor to request that a new entity be added to the game world.

* Add a new variant to `IncomingMessage` called `CreateEntities` for requesting that an entity be created. The variant should carry a field specifying how many entities should be created.
* Add a new field to outgoing messages `"created_entities"` that is a list of any entities that were just created at the request of the editor.
* The list of new entities should be sent as soon as the entities are created, ideally on the same frame.
* The list of new entities should only be sent once.

Similarly, to support destroying entities at runtime we'll need to add a variant to `IncomingMessage` called `DestroyEntities` that takes a list of entities to be destroyed. The list of destroyed entities does not need to be sent back to the editor, though.