private void btnPolozeniIspiti_Click(object sender, RoutedEventArgs e)
        {
            GlavniPanel.Children.Clear();
            PolozeniIspiti pi = new PolozeniIspiti(student);

            GlavniPanel.Children.Add(pi);
        }
        private void btnPrikaziPolozene_Click(object sender, RoutedEventArgs e)
        {
            GlavniPanel.Children.Clear();
            PolozeniIspiti p = new PolozeniIspiti(trenutniKorisnik);

            GlavniPanel.Children.Add(p);
        }