Add ExifTool backend to load metadata for image or video.

Exiv2 is always used in priority. If it fail, ExifTool is used instead.
ExifTool has the capability to create an EXV container for Exiv2. All metadata extracted with ExifTool
are concatened to the EXV container with all possible translations with Exif, Iptc, and Xmp tags.
Of course tags supported by ExifTool but not supported by Exiv2 are lost.
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