private void lbKanjiList_MouseDoubleClick(object sender, MouseEventArgs e)
        {
            if (lbKanjiList.SelectedItem != null)
            {
                string character = lbKanjiList.SelectedItem.ToString();
                Kanji  kanji     = user_pool.FindUser(user.GetMail()).FindKanji(character);

                if (tab_index == Tab.SEARCH)
                {
                    // details page
                    using (DetailsForm form = new DetailsForm(kanji))
                    {
                        form.ShowDialog();
                    }
                }
                else if (tab_index == Tab.MANAGEMENT)
                {
                    management_type = ManagementType.UPDATE_DELETE;

                    // update/delete page
                    using (AddModifyDeleteForm form
                               = new AddModifyDeleteForm(management_type, kanji, user, user_pool))
                    {
                        form.ShowDialog();
                    }
                }
            }
        }
        private void btnAddNewKanji_Click(object sender, EventArgs e)
        {
            // global management_type => enum ManagementType.ADD
            management_type = ManagementType.ADD;

            using (AddModifyDeleteForm form = new AddModifyDeleteForm(management_type, user, user_pool))
            {
                form.ShowDialog();
            }
        }