#4 Display thread/path the robot has taken

[Brian Foley]

It may be useful to add an option to display a line with arrows
showing the path the robot has taken through the maze.

This is complicated to do well, as a naďve approach will look
confusing when a robot backtracks, or goes along a piece of
corridor multiple times.

A proper implementation will probably require the maze
environment to keep track of the number of times each square has
been visited, along with some sort of list of the moves the robot
has taken.

Furthermore, an implementation of this probably implements some
code that the students are supposed to write for themselves in the
CS118 assignment 2 in Warwick, so we need to do it in a manner
that they can't use directly.