In some cases, it may be difficult to easily spot the area out of the buffer
(bad color scheme, small font, superimposed windows).
This patch adds two ncurses ui_options to bypass this problem:
- `ncurses_padding_char`, to configure the padding character,
- `ncurses_padding_fill`, to indicate whether to fill the padding line
(or to display a single character).
The default config is the legacy one (a single "~").
daa63ba879