Allow users of KPreviewJob to request a devicePixelRatio for generated thumbnails.
The result image is not guaranteed to have this ratio, and users should paint it accordingly.
Adds a convenient setDefaultDevicePixelRatio to allow an application to set its device pixel ratio preferred with a one liner patch. See dolphin's D29525
The images using a pixel ratio are stored in cache in ~/.cache/thumbnails/normal@2x for 256*256 (i.e 128*128@2x) and ~/.cache/thumbnails/large@2x for 512*512 (i.e 256*256@2x)
See kio-extras patch D29526