This revision saw lots of fiddling with the physics system, the addition
of stub files for a future plant and plant texture generation system,
and the addition of a "viewer" program that (for now) just views a
single chunk from the world. I intend to use the viewer program to test
my plant generation. I also added a DESIGN file which has some design
notes (not much for now) and added some callbacks in several places to
support the viewer functionality. Some physics bug/features were also
taken care of.

-Peter Mawhorter
This commit adds a TODO but mainly just cleans up old comments floating
around since the deprecation of frames.

-Peter Mawhorter