iOS: Hide the overlay when the keyboard is hidden

If the keyboard is hidden via the hide keyboard button then the edit
menu should also hide with it. This ensures it behaves in the same way
as native applications on iOS then.

Change-Id: I4c714dd5c5cb27d8eaf310e2911dc38feb1cb74e
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>

src/plugins/platforms/ios/qiostextinputoverlay.mm

@@ -116,6 +116,11 @@ static void executeBlockWithoutAnimation(Block block)
                 dispatch_async(dispatch_get_main_queue (), ^{ self.visible = YES; });
             }
         }];
+        [center addObserverForName:UIKeyboardDidHideNotification object:nil queue:nil
+            usingBlock:^(NSNotification *) {
+                self.visible = NO;
+        }];
+
     }
 
     return self;