For a poster generated from image.ext, suggest image-poster.pdf as filename.

Fixes #5.
Merge pull request #6 from ockham/set-default-savename

For a poster generated from img.ext, suggest img-poster.pdf as filename
Implement loading of PDFs (using poppler-qt5).

Currently only on Linux/Unix, using the build option POPPLER_QT5_LIB.