Expose reason for notification dismissal through SetInt()

On all platforms, except macOS, the reason why a notification was
dismissed is known. Specifically, it can be useful to distinguish whether
the notification closed because of a timeout, or a click by the user.

samples/dialogs/dialogs.cpp

@@ -2782,7 +2782,8 @@ private:
 
     void OnNotificationDismissed(wxCommandEvent& event)
     {
-        ShowStatus("Notification was dismissed");
+        // See wxNotificationMessage::DismissalReason for reason explanations.
+        ShowStatus(wxString::Format("Notification was dismissed (reason: %d)", event.GetInt()));
 
         Raise();