Initialise colours section with default combo colours if none present

[bdach]

Closes #30100.

This will look incorrect with argon skin because it overrides combo colours in a weird place:

osu/osu.Game/Skinning/ArgonSkin.cs

Lines 52 to 73 in 2914a5b

	Configuration.CustomComboColours = new List<Color4>
	{
	// Standard combo progression order is green - blue - red - yellow.
	// But for whatever reason, this starts from index 1, not 0.
	//
	// We've added two new combo colours in argon, so to ensure the initial rotation matches,
	// this same progression is in slots 1 - 4.
	// Orange
	new Color4(241, 116, 0, 255),
	// Green
	new Color4(0, 241, 53, 255),
	// Blue
	new Color4(0, 82, 241, 255),
	// Red
	new Color4(241, 0, 0, 255),
	// Yellow
	new Color4(232, 235, 0, 255),
	// Purple
	new Color4(92, 0, 241, 255),
	};
	}

but I'm not super sure or even willing to figure out how to access that from within the editor.

[peppy]

This will look incorrect with argon skin because it overrides combo colours in a weird place

For mapping purposes, we always want to use the default colours, not per-skin. The per-skin colours are overrides for when a user explicitly disabled beatmap colours. Although that brings up something that I may have failed to mention: the defaults aren't stored to the beatmap unless the user makes a change to them:

Jump.Desktop.2024-10-04.at.09.13.38.mp4

[bdach]

Although that brings up something that I may have failed to mention: the defaults aren't stored to the beatmap unless the user makes a change to them

That's how this PR is supposed to work - see the weird bidirectional bindings I did to the beatmap skin colours instead of straight binding, which is supposed to ensure this.

[peppy]

Okay cool, just checking before I dive in.