support item_name= in [store_items]

we use it in SotA16 to make the scenario more robust against
[modification]s that might add [item]s for decoration.

data/campaigns/Secrets_of_the_Ancients/scenarios/16_The_Mage.cfg

@@ -233,6 +233,7 @@ We crossed the high plateaus, and forded the Arkan-thoria. Then we headed up int
         x={X}
         y={Y}
         image=items/holy-water.png
+        name=holy_water
     [/item]
 #enddef
 
@@ -246,6 +247,7 @@ We crossed the high plateaus, and forded the Arkan-thoria. Then we headed up int
 
         [store_items]
             variable=holy_waters
+            item_name=holy_water
         [/store_items]
 
         [message]
@@ -307,9 +309,11 @@ We crossed the high plateaus, and forded the Arkan-thoria. Then we headed up int
         [/if]
         [remove_item]
             x,y=$x1,$y1
+			image=holy_water
         [/remove_item]
         [store_items]
             variable=holy_waters
+            item_name=holy_water
         [/store_items]
     [/event]
 

data/lua/wml/items.lua

@@ -90,6 +90,7 @@ end
 
 function wml_actions.store_items(cfg)
 	local variable = cfg.variable or "items"
+	local item_name = cfg.item_name
 	variable = tostring(variable or helper.wml_error("invalid variable= in [store_items]"))
 	wml.variables[variable] = nil
 	local index = 0
@@ -97,8 +98,14 @@ function wml_actions.store_items(cfg)
 		local items = scenario_items[loc[1] * 10000 + loc[2]]
 		if items then
 			for j, item in ipairs(items) do
-				wml.variables[string.format("%s[%u]", variable, index)] = item
+				-- note: item_name can not be part of standard location filter because
-				index = index + 1
+				--       there might be multiple items on one locations and we don't
+				--       want to return them all if only one matches the name
+				-- todo: consider making cfg.item_name a comma list or a regex.
+				if item_name == nil or item.name == item_name then
+					wml.variables[string.format("%s[%u]", variable, index)] = item
+					index = index + 1
+				end
 			end
 		end
 	end

data/schema/core/actionwml.cfg

@@ -1119,6 +1119,7 @@
 		name="store_items"
 		max=infinite
 		super="$action_wml/store_locations"
+		{SIMPLE_KEY item_name string}
 	[/tag]
 	[tag]
 		name="store_relative_direction"