UpdateCellCommand doesn't undo properly
Found this while engineering a fix for #4748 (closed), undoing a movement in the Instances list causes the Cell entry on the instance being moved to be cleared to nothing instead of replacing the previous value. Looking through apps/opencs/model/world/commands.cpp I couldn't find
mOld being set anywhere so it must just be clearing to zero.
As of current master this doesn't happen when manipulating an instance in the viewport as the cell is updated with a ModifyCommand instead.