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

[Morphior]

Does your feature request involve difficulty completing a task? Please describe.
I think it would be cool to be able to enable a vertical line separating the two columns in two-column mode to have an effect similar to a real book where the two pages which are side-by-side are separated by a crease. It would visually enhance the reading experience, especially for people who may have a hard time staying focused and whose eyes wander around. The vertical line would keep the eyes in the frame where they are currently supposed to be. Accessibility-wise it would be a great addition. It doesn't have to be a big menu entry, it can be hidden imo.

[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.