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
+    {
+    }
+}

MediaBrowser.Controller/MediaBrowser.Controller.csproj

@@ -73,6 +73,7 @@
     <Compile Include="Configuration\IServerConfigurationManager.cs" />
     <Compile Include="Dto\SessionInfoDtoBuilder.cs" />
     <Compile Include="Entities\Audio\MusicAlbumDisc.cs" />
+    <Compile Include="Entities\IByReferenceItem.cs" />
     <Compile Include="Entities\MusicVideo.cs" />
     <Compile Include="Library\ILibraryPostScanTask.cs" />
     <Compile Include="Library\ILibraryPrescanTask.cs" />