private void btnAdd_Click(object sender, RoutedEventArgs e)
        {
            Avion         noviAvion = new Avion();
            AddNEditAvion few       = new AddNEditAvion(noviAvion);
            bool?         rez       = few.ShowDialog();

            DGAvioni.Items.Refresh();
        }
        private void btnUpdate_Click(object sender, RoutedEventArgs e)
        {
            while (SelektovaniAvion())
            {
                return;
            }
            Avion         selektovaniAvion = DGAvioni.SelectedItem as Avion;
            AddNEditAvion few = new AddNEditAvion(selektovaniAvion, AddNEditAvion.EOpcija.IZMENA);

            if (few.ShowDialog() == true)
            {
                DGAvioni.Items.Refresh();
            }
        }