Replace old markdown and remove redundant events (#389)

* Remove redundant messages

* Replaced old markdown with preview.
Added nuget config to add preview feed

* Fix old extensions usage

* Scrollbar margin for the markdown.

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Co-authored-by: Burak Kaan Köse <bkaankose@outlook.com>

Wino.Mail/AppShell.xaml.cs

@@ -225,7 +225,6 @@ namespace Wino.Views
         {
             WeakReferenceMessenger.Default.Send(new ClearMailSelectionsRequested());
             WeakReferenceMessenger.Default.Send(new DisposeRenderingFrameRequested());
-            WeakReferenceMessenger.Default.Send(new ShellStateUpdated());
         }
 
         private async void MenuItemContextRequested(UIElement sender, ContextRequestedEventArgs args)