New Actor Folders objects.

- When enabled, folders are actual objects : modifying a folder won't affect actors (more compliant with OFPA).
- Enabled by default for World Partition and Level Instances.
- Can be enabled on regular levels (experimental feature 'Use Actor Folder Objects').
- If Level uses OFPA, ActorFolders will also be saved in their own package (but will use __ExternalObjects__ root folder).
- In a future refactoring, external actors will also be moved in the same root folder.
- Generalized saving object in an external package (different from its outer package)
- World outliner supports old folders and new actor folder objects : Levels using the actor folder objects will show a root Level node (like Level Instances).
#rb patrick.enfedaque, jeanfrancois.dube
#preflight 61d84b356511bc498e5cb858

#ROBOMERGE-AUTHOR: richard.malo
#ROBOMERGE-SOURCE: CL 18543443 in //UE5/Release-5.0/... via CL 18543482
#ROBOMERGE-BOT: STARSHIP (Release-Engine-Staging -> Release-Engine-Test) (v899-18417669)

[CL 18543525 by richard malo in ue5-release-engine-test branch]

Engine/Source/Editor/UnrealEd/Private/WorldFoldersImplementation.cpp

@@ -0,0 +1,19 @@
+// Copyright Epic Games, Inc. All Rights Reserved.
+
+#include "WorldFoldersImplementation.h"
+#include "WorldFolders.h"
+
+FWorldFoldersImplementation::FWorldFoldersImplementation(UWorldFolders& InWorldFolders)
+	: Owner(InWorldFolders)
+{
+}
+
+UWorld* FWorldFoldersImplementation::GetWorld() const
+{
+	return Owner.GetWorld();
+}
+
+bool FWorldFoldersImplementation::ContainsFolder(const FFolder& InFolder) const
+{
+	return Owner.FoldersProperties.Contains(InFolder);
+}