visualimagecompare: support for viewing PDFs

We generated PDFs for almost every document in our document pool.
Now, this patch allows us to display them directly in visualimagecompare.

Usage :
- launch visualcompare with the --pdf /home/of/pdfs option
- key P will switch from/to the PDF view
- buttons +/- allow to navigate in the PDF (if there is an offset between
the pages we generated and the PDF pages)

If you want that feature, you must compile visualimagecompare with poppler.