Fix incorrect use of display slot indexes

[thibaulthenry]

Sponge | SpongeAPI

In this PR I'm attempting to fix the bug I found updating display slots in API-8.

The use of the display slot indexes was wrong. So for example when I tried to display an objective to the sidebar, it actually appeared in the sidebar.aqua.

I decided to put an id field on the SpongeDisplaySlot. I tested every cases when display slots were used, and it seems to fix the bug.

Also, I noticed that some the parameters ChatFormatting color, @Nullable final Function<ChatFormatting, DisplaySlot> withColorFunction are always null, even for colored display slots.

I can update this PR to fill the color parameter, but for withColorFunction I don't know what I need to put into.
Need your feedbacks for this.

Also, I made a little change in the API, like we talked about a while ago with Zidane and Masa.

Thanks for reading this PR

[dualspiral]

Hi @thibaulthenry - I'm sorry this hasn't been looked at until now.

withColorFunction is really weird here. It's meant to be used as a function to get a DisplaySlot based on the supplied colour. For some reason, it's been made an instance method on DisplaySlot when really, it should not be.

That said, is there a use for getting an objective by colour? I'm not sure there is. If not, the withTeamColor method should be removed, and with it, the withColorFunction parameter. If so, that method should be static and (probably) call into a factory that does the selection for you.

[thibaulthenry]

Hey, I'll not be able to look at it until this week-end but yeah, I'll try to fix things with your feedbacks

[dualspiral]

Supersceded by #3405

Thanks for the initial contribution which I have taken forward (what you had was good but that "withTeamColor" thing has proved to be a bit of a headache).