Fix display of group ingredients

Commit 043f6081452365daaa033c58e0738527ccb64c3d broke a couple of things
in the course of its refactoring.  This patch restores the "G" flag that
signals group ingredients, the preference for "default:" items as group
representatives (where a representative isn't specifically registered),
and the built-in registration of default:cobble as representative for
group:stone (because it's the most commonly-used item in that group).

group.lua

@@ -1,6 +1,7 @@
 
 unified_inventory.registered_group_items = {
 	mesecon_conductor_craftable = "mesecons:wire_00000000_off",
+	stone = "default:cobble",
 	wool = "wool:white",
 }
 
@@ -21,7 +22,7 @@ end
 -- Among equally-preferred items, we just pick the one with the
 -- lexicographically earliest name.
 
-function compute_group_item(group_name)
+local function compute_group_item(group_name)
 	local candidate_items = {}
 	for itemname, itemdef in pairs(minetest.registered_items) do
 		if (itemdef.groups.not_in_creative_inventory or 0) == 0 and
@@ -40,8 +41,10 @@ function compute_group_item(group_name)
 	for _, item in ipairs(candidate_items) do
 		local pref
 		if item == unified_inventory.registered_group_items[group_name] then
+			pref = 4
+		elseif item == "default:"..group_name then
 			pref = 3
-		elseif item:gsub("^[^:]+:", "") == group_name then
+		elseif item:gsub("^[^:]*:", "") == group_name then
 			pref = 2
 		else
 			pref = 1

register.lua

@@ -156,17 +156,17 @@ local function stack_image_button(x, y, w, h, buttonname_prefix, item)
 	if name:sub(1, 6) == "group:" then
 		local group_name = name:sub(7)
 		local group_item = unified_inventory.get_group_item(group_name)
-		show_group = not group_item.sole
+		show_is_group = not group_item.sole
 		displayitem = group_item.item or "unknown"
 		selectitem = group_item.sole and displayitem or name
 	end
-	-- Hackily shift the count to the bottom right
-	local shiftstr = "\n\n        "
+	local label = string.format("\n\n%s%7d", show_is_group and "G" or "  ", count):gsub(" 1$", " .")
+	if label == "\n\n        ." then label = "" end
 	return string.format("item_image_button[%u,%u;%u,%u;%s;%s;%s]",
 			x, y, w, h,
 			minetest.formspec_escape(displayitem),
 			minetest.formspec_escape(buttonname_prefix..selectitem),
-			count ~= 1 and shiftstr..tostring(count) or "")
+			label)
 end
 
 unified_inventory.register_page("craftguide", {