Merge branch 'maintenance/ViewModels-deprecate-placeholders' into develop

The `List` content type is implicitly created as a container for nested topics in the topic graph, and is not expected to mapped directly. Collections on view models that correspond to a nested topic `List` are automatically mapped to its children, without mapping the `List` itself.

Instead, the `List` content type was exclusively introduced to support the OnTopic Editor's `QueryableAttributeDescriptor.BaseTopic` property, so that a nested topic list could be referenced as the source for e.g. a `TopicListAttributeDescriptor`, thus allowing the editor to display dropdown boxes.

This functionality can now be better served by the new `[MapAs()]` attribute (#41). This not only eliminates the need for this placeholder, but also allows binding to any arbitrary content type, even if there's not a view model associated with it. This will be implemented in OnTopic Editor 5.1.0, as tracked in OnTopicCMS/OnTopic-Editor-AspNetCore#29.

In anticipation of that, this placeholder is being marked as obsolete.

Author: JeremyCaney

OnTopic.ViewModels/TopicViewModelLookupService.cs

@@ -50,10 +50,18 @@ public TopicViewModelLookupService(IEnumerable<Type>? types = null) : base(types
       \-----------------------------------------------------------------------------------------------------------------------*/
       TryAdd(typeof(ItemTopicViewModel));
       TryAdd(typeof(ContentItemTopicViewModel));
-      TryAdd(typeof(ListTopicViewModel));
       TryAdd(typeof(LookupListItemTopicViewModel));
       TryAdd(typeof(SlideTopicViewModel));
 
+      /*------------------------------------------------------------------------------------------------------------------------
+      | Add obsolete types
+      >-------------------------------------------------------------------------------------------------------------------------
+      | These will be removed in the next major version of OnTopic.
+      \-----------------------------------------------------------------------------------------------------------------------*/
+#pragma warning disable CS0618 // Type or member is obsolete
+      TryAdd(typeof(ListTopicViewModel));
+      #pragma warning restore CS0618 // Type or member is obsolete
+
       /*------------------------------------------------------------------------------------------------------------------------
       | Add support types
       \-----------------------------------------------------------------------------------------------------------------------*/

OnTopic.ViewModels/_items/ListTopicViewModel.cs

@@ -3,6 +3,7 @@
 | Client        Ignia, LLC
 | Project       Topics Library
 \=============================================================================================================================*/
+using System;
 using System.Collections;
 
 namespace OnTopic.ViewModels {
@@ -26,6 +27,11 @@ namespace OnTopic.ViewModels {
   ///     are supplied for convenience to model factory default settings for out-of-the-box content types.
   ///   </para>
   /// </remarks>
+  [Obsolete(
+    "There should not be a need for a LookupListItem view model. If these must be referenced, prefer using e.g. [MapAs()] " +
+    "to specify a common view model.",
+    false
+  )]
   public record ListTopicViewModel: ContentItemTopicViewModel {
 
   } //Class