Sets the character to use in place of the actual text when set_visibility has been called to set text visibility to false.
i.e. this is the character used in “password mode” to show the user how many characters have been typed. By default, GTK+ picks the best invisible char available in the current font. If you set the invisible char to 0, then the user will get no feedback at all; there will be no text on the screen as they type.
a Unicode character