Show circular array as circle in Queue.py

[JMCanning78]

Show queue cells as part of a circular ring, so it is more clear how the "front" and "rear" pointers wrap around as the pass the end of the allocated array length. If possible, show both the linear array representation and the circular one on the canvas simultaneously.

If letting queue items be short strings makes it too hard to draw without overlaps, limit items to be just 2-digit numbers.

[JMCanning78]

This issue depends on issue: #23.