Add ability to sync metadata across difficulties in editor setup

[Hiviexd]

NVIDIA_Overlay_wQbbcYQ7sa.mp4

This has been a pain point in both lazer and stable, and people usually use external tools for this kind of stuff for stable.

[bdach]

This is likely completely dropping beatmap skin-related ini keys like combo colours and such.

Compare:

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

Lines 215 to 220 in 95648a3

	// save the difficulty to re-encode the .osu file, updating any reference of the old filename.
	//
	// note that this triggers a full save flow, including triggering a difficulty calculation.
	// this is not a cheap operation and should be reconsidered in the future.
	var beatmapWorking = beatmaps.GetWorkingBeatmap(b);
	beatmaps.Save(b, beatmapWorking.GetPlayableBeatmap(b.Ruleset), beatmapWorking.GetSkin());

[bdach]

Don't really get why this is a separate method. Not like anything else is going to ever call it. Probably inline it.

[bdach]

This is a potentially-destructive action that will likely never support undo. I think a confirmation dialog is warranted. Or at least some hold-to-confirm facility on the button itself to prevent accidents.