add direct setCallback method

src/de/steamwar/inventory/SWInventory.java

@@ -66,6 +66,10 @@ public class SWInventory implements Listener, Inventory {
         setItem(pos, item);
     }
 
+    public void setCallback(int pos, InvCallback c){
+        callbacks.put(pos, c);
+    }
+
     public void open(){
         player.openInventory(this);
     }