Allow selection of multiple accepted items in GUI Slots

Issue description

Just like shulker boxes, we cant put shulker boxes in shulker boxes. It would be great we can make that with custom backpacks. 

Like I want that we cant put shulkers in backpacks or small backpacks in big backpacks.

Issue comments

we cant put shulker boxes in shulker boxes

I think we can't put a shulker box inside another shulker box.