Restructuring of the files

Facepunch.Steamworks/Callbacks/ICallbackData.cs

@@ -0,0 +1,17 @@
+using Facepunch.Steamworks.Data;
+using System;
+using System.Collections.Generic;
+using System.Runtime.InteropServices;
+using System.Threading.Tasks;
+
+namespace Facepunch.Steamworks
+{
+	/// <summary>
+	/// Gives us a generic way to get the CallbackId of structs
+	/// </summary>
+	internal interface ICallbackData
+	{
+		CallbackType CallbackType { get; }
+		int DataSize { get; }
+	}
+}