private void editBtn_Click(object sender, EventArgs e)
        {
            var selectedIndex = haircutsListBox.SelectedIndex;
            var haircutForm   = new HaircutForm(HairDressingSalon.FinishedHaircuts[selectedIndex]);

            if (haircutForm.ShowDialog() != DialogResult.OK)
            {
                return;
            }
            HairDressingSalon.FinishedHaircuts[selectedIndex] = haircutForm.Haircut;
            haircutsListBox.Items[selectedIndex] = HairDressingSalon.FinishedHaircuts[selectedIndex].ToString();
        }
        private void addBtn_Click(object sender, EventArgs e)
        {
            var haircutForm = new HaircutForm();

            if (haircutForm.ShowDialog() != DialogResult.OK)
            {
                return;
            }
            var newHaircut = haircutForm.Haircut;

            HairDressingSalon.FinishedHaircuts.Add(newHaircut);
            haircutsListBox.Items.Add(newHaircut.ToString());
        }