[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
[BUGFIX] Languages argument can be left empty

In that case, the v:page.language ViewHelper will simply return the current languages uid.
Merge pull request #423 from cedricziel/noLangArg

[BUGFIX] Languages argument can be left empty
[BUGFIX] Correct order of included assets
Merge pull request #420 from bjo3rnf/assetorderfix

[BUGFIX] Correct order of included assets
Merge pull request #418 from bjo3rnf/pdfthumb

[FEATURE] Add PDF thumbnail viewhelper