Closes https://github.com/ppy/osu/issues/25608.
Logic mostly matching stable. All operations are done on `ComboOffset`
which still makes overridden combo colours weirdly relatively dependent
on each other rather than them be an "absolute" choice, but alas...
As per stable, two consecutive new combos can use the same colour only
if they are separated by a break:
52f3f75ed7/osu!/GameModes/Edit/Modes/EditorModeCompose.cs#L4564-L4571
This control is only available once the user has changed the combo
colours from defaults; additionally, only a single new combo object
must be selected for the colour selector to show up.
