Qt: Minor improvements to auto-updater (icon, show download size)

src/duckstation-qt/autoupdaterdialog.h

@@ -53,6 +53,7 @@ private:
   QNetworkAccessManager* m_network_access_mgr = nullptr;
   QString m_latest_sha;
   QString m_download_url;
+  int m_download_size = 0;
 
   bool m_display_messages = false;
   bool m_update_will_break_save_states = false;