FIXME: AActorSingleton::TryBecomeNewInstanceOrSelfDestroy WITH_EDITOR behavior

[sleeptightAnsiC]

I shall at least try to refactor this someday:

ActorSingleton/Source/ActorSingleton/Private/ActorSingleton.cpp

Lines 93 to 142 in e18772f

	#if WITH_EDITOR
	/* In case of placing an Actor in the Level Viewport, we canNOT simply Destroy it.
	* Instead, we must "tell" the Editor to delete it, which will fire some additional clean up logic.
	* Also we're showing a message to the Editor user telling what is happening, so we can avoid confusion.
	*
	* FIXME: Current implementation is fine but has few caveats:
	* 1. it "touches" the Level despite that no actual changes have been done
	* 2. if user uses 'undo' after deletion, the duplicate object will be restored
	* 3. if user's Actor does something after being placed, we won't be able to revert it
	* These are pretty bad... I should probably find another way to achieve the same goal.
	*
	* TODO: Possible solutions for the issues listed above:
	* 1. Prevent Actor from being placed into the Level in the first place
	* I have no idea if this can be achieved with Unreal Engine...
	* If that's possible, then there is probably some kind of Interface in AActor that allows it
	* but otherwise I have no clue where to look for solution
	* 2. Instead of deletion, we can simply use Editor's 'undo' feature
	* The problem with this one is that we would never know for sure how many times call the 'undo'
	* because Actor, when placed, can do some other stuff around which adds up to the undo/redo buffer.
	* We would need to find out how many times we need to call the 'undo'
	* However, this option seems the most promising and possible to implement :)
	*/
	if (ThisWorld->IsEditorWorld() && !ThisWorld->IsPlayInEditor())
	{
	/* Show Dialogue Message */
	const FText MessageTitle = this->GetMessageTitle();
	const FText MessageBody = this->GetMessageBody();
	FMessageDialog::Debugf(MessageBody, MessageTitle);
	/* Delete 'this' via UEditorActorSubsystem */
	auto* EditorActorSubsystem = GEditor->GetEditorSubsystem<UEditorActorSubsystem>();
	check(EditorActorSubsystem)
	EditorActorSubsystem->ClearActorSelectionSet();
	EditorActorSubsystem->SetActorSelectionState(this, true);
	EditorActorSubsystem->DeleteSelectedActors(ThisWorld);
	GEngine->ForceGarbageCollection(true);
	/* Garbage Actor still seems to be selected in the Details Panel despite already being destroyed.
	* 'UEditorActorSubsystem::DeleteSelectedActors' doesn't handle this by itself,
	* so we are clearing the Actor selection on the very next tick which fixes this issue. */
	ThisWorld->GetTimerManager().SetTimerForNextTick(
	[&]()->void
	{
	EditorActorSubsystem->SelectNothing();
	}
	);
	return;
	}
	#endif //WITH_EDITOR