private void DG_ShowData_MouseDoubleClick(object sender, System.Windows.Input.MouseButtonEventArgs e)
        {
            string Ids = GetSelectedCellValue();

            if (Ids != null)
            {
                int index = DG_ShowData.SelectedIndex;
                DG_ShowData.CurrentCell = new System.Windows.Controls.DataGridCellInfo(DG_ShowData.Items[index], DG_ShowData.Columns[0]);
                DG_ShowData.BeginEdit();
                var columnName = DG_ShowData.CurrentColumn.Header.ToString();

                int Id = int.Parse(Ids.ToString());
                if (columnName == "ReservationID")
                {
                    ReservationWindow AddReservation = new ReservationWindow(Id);
                    AddReservation.ShowDialog();
                }
                else if (columnName == "RoomId")
                {
                    RoomWindow AddRoom = new RoomWindow(Id);
                    AddRoom.ShowDialog();
                }
            }
            else
            {
                MessageBox.Show("Nie klikaæ po pustym Grid");
            }
        }
        private void btn_AddReservation_Click(object sender, RoutedEventArgs e)
        {
            ReservationWindow AddReservation = new ReservationWindow();

            AddReservation.ShowDialog();
        }