Add support for Container entities

Instead of using a one-cell table, it makes more sense to use a [Container][1]. 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.

[1]: http://libgdx.badlogicgames.com/nightlies/docs/api/com/badlogic/gdx/scenes/scene2d/ui/Container.html