Write metadata with ExifTool backend if Exiv2 fails to process, for exemple...

Write metadata with ExifTool backend if Exiv2 fails to process, for exemple with RAW files or Video files.
Tested successfuly with a MP4 video to insert a comment.
For RAW and other king of files as MPO or JPEG with more than 64kB of Exif segement, Exiv2 must be bannned to uses in cases of writting operations as files can be corrupted.
This require more regression tests to validate ExifTool uses in place of Exiv2.

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