Added source open/close methods and callback placeholder.

src/NTwain/Triplets/DGControl.Callback2.cs

@@ -0,0 +1,16 @@
+using NTwain.Data;
+using NTwain.Internals;
+
+namespace NTwain.Triplets
+{
+    sealed class Callback2 : BaseTriplet
+    {
+        internal Callback2(TwainSession session) : base(session) { }
+
+        public ReturnCode RegisterCallback(TW_CALLBACK2 callback)
+        {
+            return NativeMethods.DsmWin32(Session.Config.AppWin32, Session.CurrentSource.Identity,
+                DataGroups.Control, DataArgumentType.Callback2, Message.RegisterCallback, callback);
+        }
+    }
+}