private void RegistroPrestamo_Click(object sender, RoutedEventArgs e)
        {
            UsuarioTextBox.Text    = "Mario";
            ContrasenaTextBox.Text = "******";
            VentanaPrestamo ventana = new VentanaPrestamo();

            ventana.Owner = this;
            ventana.ShowDialog();
        }
        private void RegistroPrestamo_Click(object sender, RoutedEventArgs e)
        {
            VentanaPrestamo ventana = new VentanaPrestamo();

            ventana.Show();
        }