Rewrite stairs code.

- Handle stairsplus stairs and MTG stairs in the same file.
 - Only create stairs using stairsplus or MTG stairs, not both.
 - Handle map switching from MTG stairs to stairsplus automatically.
 - If using stairsplus:
   - Move stairsplus nodes to proper namespace.
   - Alias stairsplus nodes that moved to a new namespace.
   - Alias duplicated ice and snowblock stairs and slabs to their new namespace.
   - Make ice_brick sawable too.
   - Unify stairsplus cut ice and default:ice texture brightness.
 - If using MTG stairs:
   - Move stairs and slabs to MTG stairs namespace.
   - Alias stairs and slabs that moved to the new namespace.

src/nodes.lua

@@ -352,7 +352,7 @@ minetest.override_item("default:ice", {
 	param2 = 0, --param2 is reserved for how much ice will freezeover.
 	sunlight_propagates = true, -- necessary for dirt_with_grass/snow/just dirt ABMs
 	drawtype = "glasslike",
-	inventory_image  = minetest.inventorycube"default_ice.png".."^[brighten",
+	tiles = {"default_ice.png^[brighten"},
 	liquidtype = "none",
 	 -- I made this a lot harder to dig than snow blocks because ice is much more dense
 	 -- and solid than fluffy snow. ~ LazyJ
@@ -468,16 +468,3 @@ minetest.override_item("default:snow", {
 	end,
 	on_use = snow.shoot_snowball
 })
-
-
-
--- Do stairs files
-
-local path = minetest.get_modpath"snow".."/src/"
-
-dofile(path.."basic_stairs_slabs.lua")
-
-if minetest.global_exists"stairsplus"
-and minetest.get_modpath"moreblocks" then
-	dofile(path.."stairsplus.lua")
-end