A. Changes to graphical user interface:

    1.  Unquoted Tk graphics section and created a main window for the GUI.

        Refactored code for graphical user interface, splitting EnvFrame
	into EnvGUI (main Tk window), EnvToolbar, and EnvCanvas.

    2.  Disabled binding of Mouse-1 and Mouse-2, which don't do anything 
	useful.

    3.  Implemented the add-object pop-up menu.  Defined object_classes for
	each kind of environment.

    4.  Added images for ReflexVacuumAgent and Dirt, and a get_image
	method.  get_image caches images in a dictionary to prevent
	them from being garbage-collected and avoid repeated reading
	of the same files.

        Created image files for reflex vacuum agent and dirt.

B.  Changes to XYEnvironment:

    1.  Implemented find_at method.

    2.  Implemented move_to method including bumping.

C.  Other changes -- to support bumping, defined the Obstacle class,
    and made Wall a subclass of Obstacle.  A bump occurs when an agent
    tries to move to a cell occupied by any kind of Obstacle.