Set mnemonic only to hardware keycode
As discussed on https://discuss.tryton.org/t/translation-and-keyboard-shortcuts-with-alt-first-letter/4073, some char may not be available on the keyboard. So we should not use them as mnemonic.
As discussed on https://discuss.tryton.org/t/translation-and-keyboard-shortcuts-with-alt-first-letter/4073, some char may not be available on the keyboard. So we should not use them as mnemonic.