Custom ItemGroup Icon Renderers

[maximumpower55]

Title pretty much said it all. Javadocs for the new classes and methods aren't really there so would appreciate for code suggestions that add them

[lukebemish]

Yeah, that looks like a dedicated server failure. I suspect that this is going to require a fundamentally different approach to work with dedicated servers - there's a reason stuff like block render layers is set up in a map the way it is. Attaching this stuff to the object directly could work, if you have a client-only mixin that just shoves an interface onto ItemGroups, and some client side API for adding renderers to ItemGroups using that interface. Otherwise, I think you'd have to go with some sort of map

[maximumpower55]

Aurora's and lukebemish's comments have been resolved.

[maximumpower55]

A quick glance points to a need for a Checkstyle pass; Do that, and I'll continue the rest of my review

Forgot to say this yesterday but the hopefully only needed once checkstyle pass has been pushed.