Fix error when exporting a PDF with empty text areas
Port pygtk syntax in PDF exportation dialog to GI
Fix PDF output options in ocrfeeder-cli
Fix getting engine name in ocrfeeder-cli
Fix using newer versions of Unpaper

Unpaper has been forked and newer versions do not seem to work when
the output image's name is the same one as the input's so the command
had to be updated.

gb#705610
Replace print by debug for the Unpaper command
Fix problem when converting Gdk.Pixbuf to Image

The previously used method was no longer working as Pillow
would complain that "buffer is not large enough" so instead
of using the Image.frombuffer, it now uses Image.frombytes.