[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.

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My pictures folder with 3,500 JPEGs of around 12 Megapixel size, the average thumbnail generation time per image drops significantly from 80–120 ms down to as little as 3–5 ms.

This is not counting any additional processing overhead done afterwards (e.g. size to fit and what not, adding dropshadows), since the thumbnail size request is ignored in this scenario. It asks for 512x512 for me (2x scaling) and the thumbnails I have in most images are 512x384, usually.