Preserve player's search string

Keep the current inventory search string when performing inventory
form actions other than searching.  This means the player's input is
not lost if the player breaks off from entering it to switch pages,
or if the player absent-mindedly ends text input with <return>.

init.lua

@@ -8,6 +8,7 @@ unified_inventory = {}
 unified_inventory.activefilter = {}
 unified_inventory.alternate = {}
 unified_inventory.current_page = {}
+unified_inventory.current_searchbox = {}
 unified_inventory.current_index = {}
 unified_inventory.current_item = {}
 unified_inventory.crafts_table = {}