Optimize FActorFolders when there's level streaming in PIE.

This will need more investigation, but at least in PIE (to check if we can do the same in editor), when a level is streamed-in, we should only add the new missing folders for this level and when streaming out, we should remove those folders, no need to do a full rebuild.

Note : I remember something particular in this code about empty folders not being cleaned-up intentionally.

Streaming-in/out can cause hitches, see licensee description:

"We were seeing hitches inside FActorFolders::OnLevelActorListChanged, we integrated CL 25130527 Optimization to speed up rebuilding level actor list in editor, which removes the multiple occurrences during one frame, but we still get this 20ms hitch in PIE when adding or removing a level, is this needed for PIE? Wondering if this is something the editor needs, but it is not really needed while doing PIE.

I know for some other stuff events you added a if (!InWorld.IsGameWorld()), on the same file. Wondering if this would make sense too or if there is any other thing we could do to avoid hitching 20ms while on PIE."