Remove lists of if clauses

The greeter code has several ugly lists of if clauses:

        if row == self.listboxrow_text:
            tailsgreeter.utils.popover_toggle(self.text_setting.popover)
        elif row == self.listboxrow_keyboard:
            tailsgreeter.utils.popover_toggle(self.keyboard_setting.popover)
        elif row == self.listboxrow_formats:
            tailsgreeter.utils.popover_toggle(self.format_setting.popover)
        elif row == self.listboxrow_tz:
            tailsgreeter.utils.popover_toggle(self.timezone_setting.popover)

They should be replaced by some metaprogramming, making it easier to add new settings.

Feature Branch: greeter:feature/12247-replace-lists-of-if

Parent Task: #8230 (closed)

Original created by @alant on 12247 (Redmine)