Adding a new background/audio file in the editor may remove file which is used in other difficulties

[peppy]

The method for replacing the background doesn't currently check if the old background was used or not:

osu/osu.Game/Screens/Edit/Setup/ResourcesSection.cs

Lines 84 to 94 in 7cd724f

	// remove the previous background for now.
	// in the future we probably want to check if this is being used elsewhere (other difficulties?)
	var oldFile = set.GetFile(working.Value.Metadata.BackgroundFile);
	using (var stream = source.OpenRead())
	{
	if (oldFile != null)
	beatmaps.DeleteFile(set, oldFile);
	beatmaps.AddFile(set, stream, destination.Name);
	}

Same applies to audio file.

Discussed in #30435

^{Originally posted by AFasterSlowpoke October 27, 2024}
When I drag an image to the osu lazer editor to set the background, it changes the background for all difficulties, instead of just the difficulty i'm currently editing.

I think that it would improve the osu lazer editor if dragging and dropping a new background only changed the current difficulty's background, with a button to set the current difficulty's background to all other difficulties.