Turned all boot messages into clean minetest.log() calls

- Turned all the 'print' calls at server's boot to 'minetest.log' in order to
   render it more clearly

mods/stained_glass/init.lua

@@ -289,7 +289,7 @@ for h = 1, 12 do
 end  --hues
 
 
-print("[stained_glass] Loaded!")
+minetest.log("action", "[stained_glass] Loaded!")