Show threads in GUI at the bottom of the list, in specialized groups

Related to #1119

DiscordChatExporter.Gui/Converters/ChannelToGroupKeyConverter.cs

@@ -0,0 +1,30 @@
+using System;
+using System.Globalization;
+using System.Windows.Data;
+using DiscordChatExporter.Core.Discord.Data;
+
+namespace DiscordChatExporter.Gui.Converters;
+
+[ValueConversion(typeof(Channel), typeof(string))]
+public class ChannelToGroupKeyConverter : IValueConverter
+{
+    public static ChannelToGroupKeyConverter Instance { get; } = new();
+
+    public object? Convert(object value, Type targetType, object parameter, CultureInfo culture) =>
+        value switch
+        {
+            Channel channel when channel.Kind.IsThread()
+                => $"Threads in #{channel.ParentNameWithFallback}",
+
+            Channel channel => channel.ParentNameWithFallback,
+
+            _ => null
+        };
+
+    public object ConvertBack(
+        object value,
+        Type targetType,
+        object parameter,
+        CultureInfo culture
+    ) => throw new NotSupportedException();
+}