Add Custom Callbacks for SWListInv

SpigotCore_Main/src/de/steamwar/inventory/SWListInv.java

@@ -55,8 +55,10 @@ public class SWListInv<T> extends SWInventory {
     public void open(){
         opened = true;
         inventory.clear();
-        callbacks.clear();
+        callbacks.keySet().stream().filter(i -> i < 0).forEach(callbacks::remove);
-        setCallback(-999, (ClickType click) -> player.closeInventory());
+        if(!callbacks.containsKey(-999)) {
+            setCallback(-999, (ClickType click) -> player.closeInventory());
+        }
 
         if (sizeBiggerMax()) {
             if (page != 0) {