more correct EntetyItem liste maintenance

I noticed today that in a previous commit I had accidentally created a minor bug in how the "Moving" and "Mortal" entity lists are maintained in the EntityTree class.  In particular I allowed the lists to be modified while they were being iterated, thereby causing the iterator to sometimes skip an element of the list.  This doesn't cause a crash, and would rarely produce a noticeable problem, but it still needs to be fixed.