FR: be able to add a vertical line between two columns

[Morphior]

FR: be able to add a vertical line between two columns

[poire-z]

We used to have that vertical line - but somehow only on color screen, not on black&white screen.
It was removed, for consistency, in koreader/crengine#407 where I wrote:

Also removed the thin line separator that was only showing on color screen (no need for a thin line separator: the middle margin was sized to be like the other margins: a separator would cut it in half and make it look like two smaller margins).

It was discussed along #7129 where you can still see some screenshots from the time with that separator.