Add drag/drop handling

TheXamlGuy.TaskbarGroup.Core/MessageInvoker.cs

@@ -0,0 +1,14 @@
+using System.Reflection;
+
+namespace TheXamlGuy.TaskbarGroup.Core
+{
+    public class MessageInvoker : IMessageInvoker
+    {
+        public void Invoke<TMessage>(object target, TMessage message, MethodInfo methodInfo)
+        {
+            if (message is null) throw new ArgumentNullException(nameof(message));
+
+            methodInfo.Invoke(target, new object[] { message });
+        }
+    }
+}