initial scaffolding for inverted, custom message list

src/application/main-window.vala

@@ -12,5 +12,8 @@ public class MainWindow : Adw.ApplicationWindow
 
         var conversation_list_page = new NavigationPage (new ConversationListView (), "Conversations");
         split_view.sidebar = conversation_list_page;
+
+        var message_list_page = new NavigationPage (new MessageListView (new MessageListModel ("123")), "Messages");
+        split_view.content = message_list_page;
     }
 }