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

            clearClipboard();
            ElementForm form = new ElementForm(null, selectedSprava(), selectedGrupa(),
                                               clickedItem.Broj, Element.getTezina(clickedItem.Broj));

            if (form.ShowDialog() == DialogResult.OK)
            {
                tabela.addElement(form.Element, false);
                panelTabela.Invalidate();
                panelTabela.Focus();
            }
        }