Added screen collisions, w crude graphics

Introduced z-dimension and the Teltron's tilted screen.
This collects electrons at the appropriate length along the screen -
leaving a glowing dot. This way the beam shape as would be seen on the
apparatus can be constructed (unfortunately it disguises the very
interesting loops the individual electrons may take!)
Layering

Used some PGraphics magic to separate screen effects and electron
effects into different layers. Might make it easier to switch things off
and on later...