perf(memory-leaks): introduce EventSubscription.Callback property to …

…make sure it's not garbage collected before it's invoked

YDotNet/Document/Events/EventSubscription.cs

@@ -9,13 +9,23 @@ public class EventSubscription
     ///     Initializes a new instance of the <see cref="EventSubscription" /> class.
     /// </summary>
     /// <param name="id">The ID used to identify this instance of <see cref="EventSubscription" />.</param>
-    internal EventSubscription(uint id)
+    /// <param name="callback">The callback to be invoked when the related event is triggered.</param>
+    internal EventSubscription(uint id, Delegate callback)
     {
         Id = id;
+        Callback = callback;
     }
 
     /// <summary>
     ///     Gets the ID used to identify this subscription in the document.
     /// </summary>
     public uint Id { get; }
+
+    /// <summary>
+    ///     Gets the callback to be invoked when the related event is triggered.
+    /// </summary>
+    /// <remarks>
+    ///     A reference to the callback is stored to make sure it's not disposed by the GC before the unsubscription.
+    /// </remarks>
+    public Delegate Callback { get; }
 }