print with overlaps

I’ve been playing around with the print export, and one feature could make it much more usable:

allow an option to overlap the different pages by half a centimeter or so, with marker lines to make the pages easier to line up, as offered by things like rasterbator.

as it stands, the exported images are very easy to generate, but pretty hard to collate - central grid lines may be on neither sheet, alignment is very tricky, backing paper can show through gaps,…

feel free to merge this with several other small suggestions related to the print export - it’s a great new feature, but needs a few tweaks.