1.8-specific floating inventory fix (#62)

It's possible to get your inventory into a floating state where multiple virtual "inventories" are active.

This doesn't directly solve that issue, but ensures that no actions are run if the clicked inventory is not the Helper GUI.

helper/src/main/java/me/lucko/helper/menu/Gui.java

@@ -332,6 +332,11 @@ private void startListening() {
 
                     if (!isValid()) {
                         close();
+                        return;
+                    }
+
+                    if (!e.getInventory().equals(this.inventory)) {
+                        return;
                     }
 
                     int slotId = e.getRawSlot();