Added acceleration/deceleration to the image movement.
If no keys are pressed, the velocity is decreased slowly.
The factor used is 0.99*current_velocity to decrementing.
If arrow keys are pressed, the velocity is increased
in increments of 0.01.  The acceleration is in image_class
(in the position update method).
Fixed a boundary condition bug that allowed images to leave the right
side of the screen.  Now the moving image remains on the screen always.
Added routines in image_class that manage automatic movement
of images.  One of these routines is called from Mittensgui.py.
I am still working on debugging the routine that chooses a
target and moves the image automatically toward the target.