Add moving objects

[kpreid]

It should be possible to have continuously moving (i.e. like the player) objects inside a world. Such objects could be:

• Mobs
• Vehicles
• Loose objects to interact with

All such objects should be defined by worlds (just as blocks are), and may be unique (say, one-of-a-kind vehicles) or instanced like blocks (monsters, etc). It should be possible to have fine-grained collision (e.g. standing inside a boat), or would that overlap with the subworlds feature?

[kpreid]

The “physics” code has been split out into a separate class, Body, but there is no support yet for containing non-player bodies in a world or for giving bodies an appearance.