void gtk_entry_set_invisible_char ( GtkEntry* entry, gunichar ch )
Sets the character to use in place of the actual text when
gtk_entry_set_visibility() has been called to set text visibility
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.