booklet: print center line as sowing guide

[milahu]

Is your feature request related to a problem? Please describe. when stitching booklets together with a sowing machine its hard to see the center line only from the folded booklet

Describe the solution you'd like print center line as sowing guide

Limitations printer must print pages with exact margins = exact centers

Related i have implemented this feature in my own book rendering engine https://milahu.github.io/alchi/src/alchi-book/build/ (set layout: booklet) where i print a center line only on the center page of the booklet. example: booklet has 16 pages → print line between page 8 and page 9