libgimp: improve saving xmp metadata.
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Convert xmp /Iptc4xmpExt tag parts to /iptcExt because exiv2 fails when we try to use the default namespace.
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Don't only set structs from a fixed list but find all xmp array elements and check what the best struct type is: bag or seq.
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Work around a sorting issue in (g)exiv2 by using a natural sorting algorithm ourselves.
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Added some g_debug statements to make it easier to determine the cause of issues.