Show group ingredients visually in craft guide

Where a recipe specifies an ingredient by group, show a typical group
item pictorially, with a label flag to indicate that it's a group rather
than the single item.  This is the inverse of the previous arrangement,
which identified the group by label and dipicted groupiness pictorially.
The new arrangement is easier to interpret, and if the labels are ignored
it actually shows a correct input to the crafting grid.

api.lua

@@ -147,6 +147,10 @@ unified_inventory.register_craft_type("digging", {
 	height = 1,
 })
 
+function unified_inventory.register_group_representative_item(groupname, itemname)
+	unified_inventory.registered_group_representative_items[groupname] = itemname
+end
+
 function unified_inventory.register_page(name, def)
 	unified_inventory.pages[name] = def
 end