PDPDIR/src/xcss

This is a simple psychological "xperiment" program, which basically
just puts up a graphical window and then has simple commands to render
graphics into that window, collect reaction times, etc.  It is
essentially a ta/css friendly front-end to a the InterViews Canvas and
Event classes.

Aside from its own potential value, it serves as a useful
demonstration of how to write a ta/css based application that is not
PDP++.

