This adds new methods heading(), setheading(), position(),
window_width(), window_height(), setx(), and sety(), to make this more
functionality-compatible with Logo turtle graphics (Attila's last
words, not mine :-).  I had to fix the sety() code which was broken in
Attila's patch.

I'm not adopting the functionality change that Attila claimed was a
bugfix (no output without tracing), because I disagree that it's a
bug.