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.