EnvCanvas draws each object either as a canvas text (string) or using
an image from a file associated with the object class.

Implemented delete_object for Environment and XYEnvironment,
renamed XYEnvironment methods objects_at and find_at as
list_objects_at and some_objects_at, for clarity;
revised arguments lists of list_objects_at and some_objects_at;
revised implementation of bumping to work with new list_objects_at and
some_objects_at.

Promoted XYEnvironment methods list_objects_at and
some_objects_at to Environment.

Defined can_grab method for Agent class,
replacing Object method is_grabable.

Cleaned up code, removed tracing print statements, etc.

Removed -1 penalty for Nop in VacuumEnvironment.

Added Tk mainloop to test_gui, and commented out call to test_gui.

Further refactoring of EnvGUI, EnvToolbar, and EnvCanvas.

    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.