Add an instrument type for standard groupings [STYLE-936]

I like the idea, but we should make sure to get this right from the beginning. That is, please set up the thing such that we can represent the logic of the members of an ensemble on recording level:

Let us add the ensemble-instrument “string quartet” to some recording, let us add the instruments “artist A, violin”, “artist B, violin”, “artist C viola”, “artist D cello” to the recording and additionally let us set some relationship indicating that they are part of the string quartet. (I hope that this is planned, but from the written suggestion I’m not sure.) Ideally, we can also indicate the voice like “violin I”, “violin II” at that relationship (like we already can enter that the role of that baritone singer is “Papageno”)

Without that, it’s going to be a mess. This discussion is somewhat related.