Change BUTTON_INNER based on value

[bphunter1972]

From #1934, wouldn't this make a whole lot more sense in _toggle_button.py?

BUTTON_INNER: dict[bool, str] = {True: "X", False: " "}

I thought that checkboxes were simply broken, but instead it was that the bold attribute simply doesn't stand out enough at smaller fonts.

Then of course in _button():

        return Text.assemble(
            (self.BUTTON_LEFT, side_style),
            (self.BUTTON_INNER[self.value], button_style),
            (self.BUTTON_RIGHT, side_style),
        )