Add icon overlay to shelf inventory slots

mods/default/README.txt

@@ -139,6 +139,9 @@ BlockMen (CC BY-SA 3.0):
   bubble.png
   gui_*.png
 
+Wuzzy (CC BY-SA 3.0):
+  default_book_slot.png (based on default_book.png)
+
 sofar (CC BY-SA 3.0):
   default_book_written.png, based on default_book.png
   default_aspen_sapling

mods/default/nodes.lua

@@ -1800,6 +1800,18 @@ local bookshelf_formspec =
 	"listring[current_player;main]" ..
 	default.get_hotbar_bg(0,2.85)
 
+-- Inventory slots overlay
+local bx, by = 0, 0.3
+for i = 1, 16 do
+	if i == 9 then
+		bx = 0
+		by = by + 1
+	end
+	bookshelf_formspec = bookshelf_formspec ..
+		"image["..bx..","..by..";1,1;default_book_slot.png]"
+	bx = bx + 1
+end
+
 minetest.register_node("default:bookshelf", {
 	description = "Bookshelf",
 	tiles = {"default_wood.png", "default_wood.png", "default_wood.png",