Add support for Container entities

[mk12]

Instead of using a one-cell table, it makes more sense to use a Container. I've added container*, container, container!, and container? to match all the other types.

The reason I wanted to add this: I was trying to put entities in the corners of the screen, but with a margin. The simplest way to do this is to put everything in a container and then :pad the container.

[oakes]

Thanks, it looks good. I assume you didn't mean to make container* call (ActorEntity. (Table. child)) right?

[mk12]

Not sure why I did that — yes, should be (ActorEntity. (Container. child)).

[oakes]

No problem, I merged and fixed it. Thanks!