moved IByReferenceItem to the controller project

MediaBrowser.Controller/Entities/IByReferenceItem.cs

@@ -0,0 +1,12 @@
+
+namespace MediaBrowser.Controller.Entities
+{
+    /// <summary>
+    /// This is a marker class that tells us that a particular item type may be physically resolved
+    /// more than once within the library and we need to be sure to resolve them all to the same
+    /// instance of that item.
+    /// </summary>
+    public interface IByReferenceItem
+    {
+    }
+}