Self-Moving Actor Demo (0.15.1 Alpha)

This update adds a demonstration of a self-moving Actor: the Attraction Ball.  It's attracted to ferrous materials, like iron and steel blocks.  It's also attracted to other Attraction Balls and ferrous Actors.  Likewise, ferrous Actors will be attracted to it.  I added an iron ball to demonstrate this.

Here's an animated gif of it in action: