file-info: Add a set of attributes for large thumbnails

Some applications (eg., gnome-photos) really want a large thumbnail,
if one can be created. Simply falling back to a smaller one (probably
created by an old nautilus), without giving the application a chance
to create a bigger thumbnail, is undesirable because they will appear
fuzzy.

Therefore, at separate attribute sets for all the thumbnail sizes
that are supported in the spec: normal/large/x-large/xx-large.

The old attribute will now return by default the biggest available, as
it used to be, but also including the x-large and xx-large cases.

Co-Authored-by: Marco Trevisan <mail@3v1n0.net>
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