Added support for sync notification handlers in the event aggregator

src/Umbraco.Core/Events/INotificationHandler.cs

@@ -12,6 +12,20 @@ namespace Umbraco.Core.Events
     /// <typeparam name="TNotification">The type of notification being handled.</typeparam>
     public interface INotificationHandler<in TNotification>
         where TNotification : INotification
+    {
+        /// <summary>
+        /// Handles a notification
+        /// </summary>
+        /// <param name="notification">The notification</param>
+        void Handle(TNotification notification);
+    }
+
+    /// <summary>
+    /// Defines a handler for a async notification.
+    /// </summary>
+    /// <typeparam name="TNotification">The type of notification being handled.</typeparam>
+    public interface INotificationAsyncHandler<in TNotification>
+        where TNotification : INotification
     {
         /// <summary>
         /// Handles a notification