/// <summary>
/// Create Invoice
/// </summary>
/// <param name="sender"></param>
/// <param name="e"></param>
private void Btn_Create_Click(object sender, RoutedEventArgs e)
{
MI_Search.IsEnabled = false;
MI_Close.IsEnabled = true;
Btn_Create.IsEnabled = false;
Btn_Edit.IsEnabled = false;
Btn_Delete.IsEnabled = true;
Btn_Edit.Visibility = Visibility.Hidden;
Btn_Save.Visibility = Visibility.Visible;
Btn_Delete.Visibility = Visibility.Hidden;
Btn_Cancel.Visibility = Visibility.Visible;
dp_InvoiceDate.IsEnabled = true;
cb_InvoiceItems.IsEnabled = true;
btn_AddToInvoice.IsEnabled = true;
btn_RemoveFromInvoice.IsEnabled = true;
tb_InvoiceItemsCost.Text = "";
DG_Items.ItemsSource = null;
Main.Visibility = Visibility.Hidden;
Invoice.Visibility = Visibility.Visible;
Search.Visibility = Visibility.Hidden;
Item.Visibility = Visibility.Hidden;
cb_InvoiceItems.ItemsSource = mn.GetLineItems();
cb_InvoiceItems.SelectedIndex = -1;
dp_InvoiceDate.IsEnabled = true;
lbl_InvoiceNumber.Content = "Invoice #: TBD";
dp_InvoiceDate.SelectedDate = null;
dp_InvoiceDate.DisplayDate = DateTime.Today;
}