Added IRegionCreator interface that all ApplicationPlugins that are creators of Scenes should implement and register with the ApplicationRegistry.StackModuleInterface<>(); So that other plugins can attach to their OnNewRegionCreated event.

Made some changes to IRegistryCore and RegistryCore so they support "Stacked" interfaces.

OpenSim/Framework/IRegionCreator.cs

@@ -0,0 +1,12 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+
+namespace OpenSim.Framework
+{
+    public delegate void NewRegionCreated(IScene scene);
+    public interface IRegionCreator
+    {
+        event NewRegionCreated OnNewRegionCreated;
+    }
+}