CombinedInventoryProxy for double chests is now ephemeral

previously we had to make sure that the two chests shared the same instance, so that the viewer
lists would be properly updated. Now, instead, we can just combine the viewer lists of the individual
chests, which is a lot cleaner.

src/inventory/transaction/SlotChangeActionBuilder.php

@@ -117,4 +117,20 @@ final class SlotChangeActionBuilder extends BaseInventory{
 		}
 		return $result;
 	}
+
+	public function getViewers() : array{
+		return [];
+	}
+
+	public function removeAllWindows() : void{
+		//NOOP
+	}
+
+	public function onOpen(InventoryWindow $window) : void{
+		//NOOP
+	}
+
+	public function onClose(InventoryWindow $window) : void{
+		//NOOP
+	}
 }