Add generalized preloading

[kpreid]

There a number of things which we could preload incrementally to avoid delays the first time the player takes an action. It would be nice to have a general framework to plug in such processes so that we don't have to add a new hook in the main loop or step() or an independent setTimeout() for each, they all get progress bars if desirable, and they don't all run at once and bog down the frame rate. Candidates:

• Sound synthesis.
• Block particle geometry calculation.
• Block renders for the block worlds' block sets.

Additionally, where possible we should look into running these calculations in background threads (Web Workers).