[scribus] Horizontally center an image frame on a "fold" or "page"

[Peter Nermander]

> - activate snapping to guidelines
> - add a rectangle going from one guideline to the the other
> - use window > align and distribute to center the image in relation to
>  the shape
> - remove the shape

I was just going to suggest the same approach, but I would probably
leave the shape in place (and have one shape for each "pane"). It can
be of use for more things and an empty shape won't end up in the PDF
anyway, will it?

/Peter