<:> Mouse and keyboard interaction

If we want the applet to be interactive, we need to add MouseListener functionality.

This involves defining methods which `listen' out for certain types of button-press.

For full details, look at the documentation for `MouseListener'. (This is most easily done by googling `MouseListener', or `Java 5.0 MouseListener' if you want specific documentation for version 5, say.) Look at the documentation for `KeyListener' to see how to make an applet listen to the keyboard.