Add drag/drop handling

TheXamlGuy.TaskbarGroup.Core/IAsyncMessageHandler.cs

@@ -1,11 +1,16 @@
 namespace TheXamlGuy.TaskbarGroup.Core
 {
-    public interface IAsyncMessageHandler<TMessage>
+    public interface IAsyncMessageHandler
+    {
+
+    }
+
+    public interface IAsyncMessageHandler<TMessage> : IAsyncMessageHandler
     {
         Task Handle(TMessage message, CancellationToken canellationToken = default);
     }
 
-    public interface IAsyncMessageHandler<TReturn, TMessage>
+    public interface IAsyncMessageHandler<TReturn, TMessage> : IAsyncMessageHandler
     {
         Task<TReturn> Handle(TMessage message, CancellationToken cancellationToken = default);
     }