Allow selecting/opening a source by id in case of duplicate names.

src/NTwain/DataSource.cs

@@ -103,6 +103,14 @@ namespace NTwain
 
         internal TWIdentity Identity { get; private set; }
 
+        /// <summary>
+        /// Gets the id of the source.
+        /// </summary>
+        /// <value>
+        /// The identifier.
+        /// </value>
+        public int Id { get { return Identity.Id; } }
+
         /// <summary>
         /// Gets the source's product name.
         /// </summary>