Tagging interface for event handlers

Support events in the TorchAPI
Static registry, non-static registration methods.
Support unregister by assembly.

Torch.API/Managers/Event/IEvent.cs

@@ -0,0 +1,11 @@
+namespace Torch.API.Managers.Event
+{
+    public interface IEvent
+    {
+        /// <summary>
+        /// An event that has been cancelled will no be processed in the default manner.
+        /// </summary>
+        /// <seealso cref="EventHandlerAttribute.SkipCancelled"/>
+        bool Cancelled { get; }
+    }
+}