Accept both WASD and arrow keys to control dude in ServerClientDemo.
Add BlockDynamics.  Don't do much with it, yet.
Super crappy (but improveable?) physics framework in place.

I think the idea is that this ~could in theory~ be more efficient
than doing physics with block behaviors.

Create objects for bit address ranges and block and node instances.
Note CoAP in robot control protocol doc.
Add TODO doc.
Add a nice line-ending at the end of TODO.
Note dynamic entity physics for game19 in TODO.
Refactor Game19 WorldNode classes.

Replace isLeaf() with getNodeType() and
rename Leaf/Branch node classes as BlockStack/QuadTree.
Shade opaque cells more nicely, such that opaque ones at the screen edge are shaded.
Merge BlockStack and BlockStackNode objects.
Remove commented-out obsolete functions in VisibilityChecker.