Add snooze support for calendar reminders (toast UI, service, DB) (#825)

* Filter reminder snooze options by default reminder

* Some updates.

* Fixing empty welcome page issue and attendee loading.

* Icon system for notifications and snooze options etc.

Wino.Mail.WinUI/MailAppShell.xaml.cs

@@ -38,6 +38,8 @@ public sealed partial class MailAppShell : MailAppShellAbstract,
     IRecipient<CreateNewMailWithMultipleAccountsRequested>,
     IRecipient<InfoBarMessageRequested>
 {
+    public Frame GetShellFrame() => InnerShellFrame;
+
     [GeneratedDependencyProperty]
     public partial UIElement? TopShellContent { get; set; }