Object layering for beatmaps (mainly for osu!mania)

[Rekkonnect]

There are some cases where in difficult osu!mania beatmaps the notes are following multiple layers of the song. In these cases, a number of layers for the notes would be useful. May be also used for isolating parts of the beatmap that you may need to mark as done, rather than just having the lock note feature for that which does not show whether an object is locked (in osu!stable at least).

I don't think this feature should be used for any other gamemode since they do not support multiple actions at the same time. However I consider this a useful feature for the cases of creating a difficult beatmap with several channels of streams or anything.

I do realize this is far from being implemented any time soon, but I decided to drop this suggestion here for having it, or try to work it out myself if I figure out the way to implement it in the project.