add support for group `air_flowable`.

replace the placeholder api functionality for checking whether air flows
through a node, with something that relies on a dedicated group (instead
of a hard-coded list in the api code).

mods/bg_api/util_node.lua

@@ -11,20 +11,13 @@ end
 
 
 
--- probably temporary; will probably use groups for this later.
-local air_flowable = {
-	"air",
-	"bg_tree:leaves",
-	"bg_tree:leaves_alive",
-}
-
 function blockgame.air_flows_through (pos, node)
 	node = node or minetest.get_node(pos)
 	local node_name = node.name
-	for _, name in pairs(air_flowable) do
-		if name == node_name then return true end
-	end
-	return false
+
+	if node_name == "air" then return true end
+
+	return minetest.get_item_group(node_name, "air_flowable") > 0
 end