Thumbnails: Make thumbnail generation devicePixelRatio aware

Use the metadata devicePixelRatio to scale up dimensions of thumbnails generated.
Sets the devicePixelRatio on the output QPixmap according to the thumbnail generated.
Make the Text Thumbnail creator devicePixelRatio aware.
Follows xdg thumbnail specs 0.9 https://specifications.freedesktop.org/thumbnail-spec/thumbnail-spec-latest.html

BUG: 342053
FIXED-IN: 21.08