How can we add scrolling to editor 2.1 (#185)? We need to update scrollX and scrollX as soon as the cursor moves out of the screen.

So it's something like this:

  if curX < scrollX then scrollX = curX end
  if curY < scrollY then scrollY = curY end
  if curX >= scrollX+displayCols then scrollX = curX-displayCols end
  if curY >= scrollY+displayRows then scrollY = curY-displayRows end
  
Now we just insert that at the appropriate place in the source, and we're done.

Note: Did it, see #192 (now also automatically included in #185).