ECS - Command Buffers

[foodelevator]

Add a way to buffer some actions to be performed on the World at a later stage when possible.

This will make it possible to for example remove certain entities while looping over them.

Todo:

• [x] Command to spawn & despawn entities
• [x] Command to add components to entities
• [x] Add a way to add components to newly spawned entities
• [x] Add a way to get a handle to the new Entity being spawned

[foodelevator]

The current API when creating a Commands is not very nice, but there were some borrow checking problems that had to be solved.