XYEnvironment.  Identified three possible inelegances or errors in the 
__init__ method of EnvCanvas.

of the NQueensProblem -- it had been ignoring conflicts with the
last queen previously added.

Changed 'Delay' slider to 'Speed' slider;

Changed self.running values to boolean True, False.

for more uniform initialization of Agents.

Improved GUI with commands to list objects and agents.

Fixed GUI speed control (delay slider), narrowed the
range and increased the resolution.

Added three new image files and image credits file.
Better image for dirt

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.