Don't allow selecting non-element types when composing element types

src/Umbraco.Web/Models/ContentEditing/GetAvailableCompositionsFilter.cs

@@ -16,5 +16,10 @@
         /// along with any content types that have matching property types that are included in the filtered content types
         /// </summary>
         public string[] FilterContentTypes { get; set; }
+
+        /// <summary>
+        /// Wether the content type is currently marked as an element type
+        /// </summary>
+        public bool IsElement { get; set; }
     }
 }