Mark toasts as read when closing the overlay for added safety

I'm not sure how the read status will play out going forward so I'm just
adding this to keep things conforming for now.

osu.Game/Overlays/NotificationOverlay.cs

@@ -230,6 +230,7 @@ namespace osu.Game.Overlays
         private void markAllRead()
         {
             sections.Children.ForEach(s => s.MarkAllRead());
+            toastTray.MarkAllRead();
             updateCounts();
         }
     }

osu.Game/Overlays/NotificationOverlayToastTray.cs

@@ -75,6 +75,12 @@ namespace osu.Game.Overlays
             };
         }
 
+        public void MarkAllRead()
+        {
+            toastFlow.Children.ForEach(n => n.Read = true);
+            InternalChildren.OfType<Notification>().ForEach(n => n.Read = true);
+        }
+
         public void FlushAllToasts()
         {
             foreach (var notification in toastFlow.ToArray())