[JPEG Thumbnailer] Read thumbnail from EXIF metadata, if available

It's a lot more efficient to read the thumbnail stored within the image
than loading the full image and scaling it down, even though Qt's JPEG
handler and libjpeg do some optimizations and fast downscaling already.
3 jobs for !163 with work/kbroulik/exif-jpeg-thumbnail in 9 minutes and 11 seconds (queued for 3 minutes and 13 seconds)
latest merge request