Grouped membership event splitting

With respect to this comment:

https://github.com/alphapapa/ement.el/blob/b8356417ffce3c47cb5b1e2a338d42add17d5f29/ement-room.el#L3023-L3024

I think the answer is that the grouped membership change event needs to get split. I.e.:

1. Order by earliest/latest, it doesn't matter.
2. After inserting the new event, check if a grouped membership event immediately before/after (when ordering by earliest/latest) "straddles" the event. If so, split it.