Added possibility to chain includes. (#331)

* Added possibility to chain includes.

* Removed interface.

* Removed the need for generating GUIDs as ids and added tests.

src/ApplicationCore/Interfaces/IIncludeQuery.cs

@@ -0,0 +1,16 @@
+using Microsoft.eShopWeb.ApplicationCore.Helpers.Query;
+using System.Collections.Generic;
+
+namespace Microsoft.eShopWeb.ApplicationCore.Interfaces
+{
+    public interface IIncludeQuery
+    {
+        Dictionary<IIncludeQuery, string> PathMap { get; }
+        IncludeVisitor Visitor { get; }
+        HashSet<string> Paths { get; }
+    }
+
+    public interface IIncludeQuery<TEntity, out TPreviousProperty> : IIncludeQuery
+    {
+    }
+}