private void numNumOfStroke_Enter(object sender, EventArgs e)
 {
     txtKana.Text = "";
     Placeholder.leave_event_placeholder(txtKana, "Hiragana/Katakana");
     txtMeaning.Text = "";
 }
 private void txtKana_Leave(object sender, EventArgs e)
 {
     Placeholder.leave_event_placeholder(txtKana, "Hiragana/Katakana");
 }