Read the /etc/shells and looks for installed shells to
        fill the default_shell combobox.

def populate_shell_combo(self):
        """Read the /etc/shells and looks for installed shells to
        fill the default_shell combobox.
        """
        cb = self.get_widget('default_shell')
        # append user shell as first option
        cb.append_text(USER_SHELL_VALUE)
        if os.path.exists(SHELLS_FILE):
            lines = open(SHELLS_FILE).readlines()
            for i in lines:
                possible = i.strip()
                if possible and not possible.startswith('#') and os.path.exists(possible):
                    cb.append_text(possible)

        for i in get_binaries_from_path(PYTHONS):
            cb.append_text(i)