Show notification on first run after an update with a link to the changelog

Closes #487

DiscordChatExporter.Gui/Services/SettingsService.cs

@@ -1,4 +1,5 @@
-using DiscordChatExporter.Core.Exporting;
+using System;
+using DiscordChatExporter.Core.Exporting;
 using Microsoft.Win32;
 using Tyrrrz.Settings;
 
@@ -20,6 +21,8 @@ public partial class SettingsService : SettingsManager
 
     public bool ShouldReuseAssets { get; set; }
 
+    public Version? LastAppVersion { get; set; }
+
     public string? LastToken { get; set; }
 
     public ExportFormat LastExportFormat { get; set; } = ExportFormat.HtmlDark;