V9: Place notifications in the same namespace (#10231)

* Gather all notifications in Umbraco.Cms.Core.Notifications

* Rename notifications to match convention

* Move and rename missed notifications

* Move the three remaining public notification into Umbraco.Cms.Core.Notifications

src/Umbraco.Core/Notifications/SavingNotification.cs

@@ -0,0 +1,21 @@
+// Copyright (c) Umbraco.
+// See LICENSE for more details.
+
+using System.Collections.Generic;
+using Umbraco.Cms.Core.Events;
+
+namespace Umbraco.Cms.Core.Notifications
+{
+    public abstract class SavingNotification<T> : CancelableEnumerableObjectNotification<T>
+    {
+        protected SavingNotification(T target, EventMessages messages) : base(target, messages)
+        {
+        }
+
+        protected SavingNotification(IEnumerable<T> target, EventMessages messages) : base(target, messages)
+        {
+        }
+
+        public IEnumerable<T> SavedEntities => Target;
+    }
+}