private void mnPromeniElement_Click(object sender, EventArgs e)
        {
            if (clickedItem == null || clickedItem.Element == null)
            {
                return;
            }

            clearClipboard();
            Element     element = clickedItem.Element;
            ElementForm form    = new ElementForm(element.Id, element.Sprava, element.Grupa,
                                                  element.Broj, element.Tezina);

            if (form.ShowDialog() == DialogResult.OK)
            {
                tabela.promeniElement(element, form.Element);
                clickedItem.Element = form.Element;
                panelTabela.Invalidate();
                panelTabela.Focus();
            }
        }