make it possible not to include  user categories (@foo) in content server field lists.
Merge from trunk
Correct implementation of ignorable_field_keys
Read calibre book uuid when reading metadata from MOBI files, needed for round-tripping to preserve the uuid when tweaking
ebook-polish: Roundtripping for both epub and azw3.
ebook-polish: Initial subsetting code
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Keep ids of elements in ncx the same when roundtripping azw3
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report font name rather than filename