Show item usages in craft guide

When the craft guide is showing a craft, the output slot is now a button,
which causes the craft guide to show ways in which that output can be
used.  This mirrors the way input slots are buttons that show recipes
for the selected ingredient.  Usages of an item can be iterated through
in the same way as recipes for the item.  This incidentally offers some
ability to retrace one's steps through a crafting chain, without storing
actual history.

init.lua

@@ -6,14 +6,15 @@ local worldpath = minetest.get_worldpath()
 -- Data tables definitions
 unified_inventory = {}
 unified_inventory.activefilter = {}
+unified_inventory.active_search_direction = {}
 unified_inventory.alternate = {}
 unified_inventory.current_page = {}
 unified_inventory.current_searchbox = {}
 unified_inventory.current_index = {}
 unified_inventory.current_item = {}
+unified_inventory.current_craft_direction = {}
 unified_inventory.registered_craft_types = {}
-unified_inventory.crafts_table = {}
-unified_inventory.crafts_table_count = 0
+unified_inventory.crafts_for = { usage = {}, recipe = {} }
 unified_inventory.players = {}
 unified_inventory.items_list_size = 0
 unified_inventory.items_list = {}