fix for non-animating models

Animating models need ModelEntityItem::update() called every so often.  This means animating entities need to be categorized as "Moving" in the EntitySimulation.
No, because the old code would only return Moving when the entity had been categorized as Static.  There is a third category (Mortal) that would not be handled.  I just simplified the code to always return Moving if it needs animation, otherwise defer to the base class's calculation -- no special logic pivot on Static.