Allow plugins to set fake "display groups"

[Xd-pro]

I need nickname functionality for my server, and in order for it to be effective, it should make the nicknamed player look like a default rank. A good way to implement this would be to allow plugins to choose a display group. If it not set or set to null, then it should use the real group of the player. Otherwise, the chat format and name tag of the display group should be used instead.