Add fading anim to graph

Previous commits established a graphic display that left items on the grid
until the screen was manually refreshed.  This commit modified display so that
items fade away gradually over the course of 10 seconds (or 5 refresh periods).

In a final product, a more acceptable solution would involve allowing the user
to decide whether shots fade or are kept imprinted on the map.  This can be
done using a simple 'check-box'.