Add four semantic integration visual hints:

- Error bars: show red bars at the left of lines when the command fails.
- Error background: show dark red background for failed commands.
- Alternating bars: show gray bars for every odd command.
- Alternating background: show dark gray background for every odd command.

Each of those is configurable in the profile individually. The bars are
enabled by default. The backgrounds show when visual hints keys are pressed
by default.