add a choice input line

def add_choice(self, setting, choices):
        '''add a choice input line'''
        tab = self.panel(setting.tab)
        default = setting.value
        if default is None:
            default = choices[0]
        ctrl = wx.ComboBox(tab, -1, choices=choices,
                           value = str(default),
                           style = wx.CB_DROPDOWN | wx.CB_READONLY | wx.CB_SORT )
        self._add_input(setting, ctrl)