LevelEditor: "Replace Actor" marks level as dirty even when using external actors

When using external actors, adding/removing/replacing/editing an actor instance inside a level does not cause any modification to the Level package, which helps reduce contention for the map file on a multi-user workflow.

When a Level Blueprint is present, removing an actor causes the map to be flagged as "modified" only if the removed actor is referenced by the Level Blueprint. This is also the expected behavior when replacing an actor with another by using the "Replace selected actors with..." feature from the popup menu. However, when replacing actors in this way, the map file is currently being marked as dirty whenever a Level Blueprint is present, even if it does not reference the affected actors. This unneccessarily negates the benefit of One-File-Per-Actor on reducing contention for the map file.

The behavior seems to be caused by the call chain below. The final Modify() is called on the Level Blueprint even when there are no references to be replaced.

FLevelEditorActionCallbacks::ReplaceActors()
UEditorEngine::ReplaceSelectedActors()
UEditorEngine::ReplaceActors()
FBlueprintEditorUtils::ReplaceAllActorRefrences()
InLevelScriptBlueprint->Modify();

• Create a new world-partition level, or create a new basic level and enable external actors. Save it.
• Place any actor on the level (e.g. a Cube)
• Right-click the placed actor, select "Replace Selected Actors with", and choose a different actor (e.g. a Sphere).
• Note that the map asset was not marked as modified so far.
• Select "Open Level Blueprint" from the toolbar. Add a PrintString node to BeginPlay. Compile, save and close.
• Right-click the previous actor, select "Replace Selected Actors with", and choose a different actor (e.g. a Cone).
• Note that the map asset was unneccessarily marked as modified.

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