private void ClickAddComputer(object sender, RoutedEventArgs e)
        {
            AddComputer addpc = new AddComputer();

            if (addpc.ShowDialog() == true)
            {
                computers.Add(addpc.computer);
            }
        }
        private void ClickUpdateComputer(object sender, RoutedEventArgs e)
        {
            var sel = HardwareListView.SelectedItems;

            foreach (var item in sel)
            {
                AddComputer addpc = new AddComputer(item as Computer);
                addpc.ShowDialog();
            }
            HardwareListView.ItemsSource = null;
            HardwareListView.ItemsSource = computers;
        }