ContentTypeService GetAllElementTypes method

Added an extension method to get all the Element Types from
the ContentTypeService.

Refactored NestedContent's controller accordingly.

src/Umbraco.Core/Services/ContentTypeServiceExtensions.cs

@@ -7,6 +7,21 @@ namespace Umbraco.Core.Services
 {
     public static class ContentTypeServiceExtensions
     {
+        /// <summary>
+        /// Gets all of the element types (e.g. content types that have been marked as an element type).
+        /// </summary>
+        /// <param name="contentTypeService">The content type service.</param>
+        /// <returns>Returns all the element types.</returns>
+        public static IEnumerable<IContentType> GetAllElementTypes(this IContentTypeService contentTypeService)
+        {
+            if (contentTypeService == null)
+            {
+                return Enumerable.Empty<IContentType>();
+            }
+
+            return contentTypeService.GetAll().Where(x => x.IsElement);
+        }
+
         /// <summary>
         /// Returns the available composite content types for a given content type
         /// </summary>

src/Umbraco.Web/PropertyEditors/NestedContentController.cs

@@ -1,5 +1,6 @@
 using System.Collections.Generic;
 using System.Linq;
+using Umbraco.Core.Services;
 using Umbraco.Web.Editors;
 using Umbraco.Web.Mvc;
 
@@ -11,8 +12,8 @@ namespace Umbraco.Web.PropertyEditors
         [System.Web.Http.HttpGet]
         public IEnumerable<object> GetContentTypes()
         {
-            return Services.ContentTypeService.GetAll()
-                .Where(x => x.IsElement)
+            return Services.ContentTypeService
+                .GetAllElementTypes()
                 .OrderBy(x => x.SortOrder)
                 .Select(x => new
                 {