feat(Core/Grids): Implement visibility notifier (#15919)

* Cherry-picked from TrinityCore (unable to find author)

src/server/game/Grids/ObjectGridLoader.h

@@ -53,6 +53,23 @@ private:
     uint32 i_corpses;
 };
 
+//Stop the creatures before unloading the NGrid
+class AC_GAME_API ObjectGridStoper
+{
+public:
+    void Visit(CreatureMapType& m);
+    template<class T> void Visit(GridRefMgr<T>&) { }
+};
+
+//Move the foreign creatures back to respawn positions before unloading the NGrid
+class AC_GAME_API ObjectGridEvacuator
+{
+public:
+    void Visit(CreatureMapType& m);
+    void Visit(GameObjectMapType& m);
+    template<class T> void Visit(GridRefMgr<T>&) { }
+};
+
 //Clean up and remove from world
 class ObjectGridCleaner
 {