[poppler] something like an "image_list" API for cpp frontend

[Jeroen Ooms]

> El dilluns, 2 d’abril de 2018, a les 10:22:51 CEST, suzuki toshiya va escriure:
> > if it is not the time to put "image_list" into cpp frontend
> It is ok, actually i know someone else that wanted to do that.

FWIW, if Suzuki is still interested, I would be very happy to extract
images via the c++ api as well. Somthing like 'pdfimages' is a common
feature request in the R package pdftools (which wraps poppler-cpp).

Thanks!