implement nextEntityID save/load

src/objects/Entities.hpp

@@ -192,9 +192,17 @@ public:
     void despawn(entityid_t id);
     dynamic::Value serialize(const Entity& entity);
 
+    void setNextID(entityid_t id) {
+        nextID = id;
+    }
+
     inline size_t size() const {
         return entities.size();
     }
+
+    inline entityid_t peekNextID() const {
+        return nextID;
+    }
 };
 
 #endif // OBJECTS_ENTITIES_HPP_