Added todo file.
Continued to add items.  They're currently rendered by sticking them
on the canvas before vis lines are drawn in the refresh.  It actually
works pretty well.  There is a bug that when you're moving to the left
away from it, the items disappear past a certain point.  There may be an
error in my in_range() function that just hasn't been an issue before.  I'll
look into it.