World items

Feature complete.
Items drop into the world and are picked up by players that walk near them.
Tons of refactoring happened to make this possible.
Hmm... I have to think about this.

This approach makes me slightly apprehensive, because it doesn't seem like it would scale well as we get more and more types of things in the world. We don't really want each one to have it's own set of listeners, own protocol handling code, etc.

It would perhaps be reasonable to have BioticListeners and EntityListeners, since Biotics do get more events, but they you're going to have to filter duplicates somehow.

Hmm...