gui: Use whline() for drawing the separator.

This should be slightly more efficient.

This patch also changes the type of struct gui_theme->sep_str from
const char * to plain char and renames the field to sep_char. It must
be a single-character string anyway.

For the colorful blackness theme we now use the default character
from curses.h rather than '-'. This looks nicer. Tested with ncurses
on Linux and the curses implementation that ships with NetBSD.