Refactor subscribe and unsubscribe to support MQTTv5.

Source/MQTTnet/IApplicationMessageReceiver.cs

@@ -1,9 +1,13 @@
 using System;
+using MQTTnet.Client.Receiving;
 
 namespace MQTTnet
 {
     public interface IApplicationMessageReceiver
     {
+        IMqttApplicationMessageHandler ReceivedApplicationMessageHandler { get; set; }
+
+        [Obsolete("Use _ReceivedApplicationMessageHandler_ instead.")]
         event EventHandler<MqttApplicationMessageReceivedEventArgs> ApplicationMessageReceived;
     }
 }