Add InventoryFillerCommand.gui

Closes: #27
Fix PistonCalculator movement

SpigotCore/SpigotCore_Main/src/de/steamwar/inventory/SWInventory.java

@@ -86,6 +86,10 @@ public class SWInventory implements Listener {
         }
     }
 
+    public boolean hasItem(int pos) {
+        return inventory.getItem(pos) != null;
+    }
+
     public void setItem(int pos, SWItem item) {
         setItem(pos, item.getItemStack(), item.getCallback());
     }

SpigotCore/SpigotCore_Main/src/de/steamwar/inventory/SWItem.java

@@ -220,4 +220,9 @@ public class SWItem {
         itemStack.setItemMeta(itemMeta);
         return this;
     }
+
+    public SWItem setAmount(int count) {
+        itemStack.setAmount(count);
+        return this;
+    }
 }