Slightly more featureful than regexdemo.py.

Particles break out of their loop when the main loop exits.

Run 500 steps instead of forever.

demo, so here it is.

explanatory message when the manual directories aren't found.
(I have to say I'm surprised that it actually still works!)

	arguments and explicit calls to .pack() are used; no more dictionaries
	are being passed to Tkinter constructors.  Otherwise, the example is
	unchanged.  (The app isn't implemented as a Python object.)