[FEATURE] Add PDF thumbnail viewhelper
[TASK] Add "pdf" prefix to generated PDF thumbnails

This adds a dead-simple pdf prefix to the generated filename.
That way, one is able to simply delete all vhs-pdf-* files
from typo3temp without much hassle after fiddeling with ImageMagick
Merge pull request #418 from bjo3rnf/pdfthumb

[FEATURE] Add PDF thumbnail viewhelper