Adopt libexiv2 or ExifTool for metadata handling

[Beep6581]

Metadata handling in RawTherapee is far from optimal. I rip out and process chunks of metadata from ExifTool and manually merge them into the rtexif/* files. This is tedious and error prone, and in all the years that RT has been open-source I'm the only person to have done that. This issue is to adopt libexiv2 or ExifTool to outsource our metadata handling needs. Although this will initially take considerable effort, in the long run it should result in less work when it comes to having up to date camera and lens info, and adding support for reading and writing metadata to various file formats.

Robin of Exiv2 was willing to help in some capacity with libexiv2 adoption. *3

Furthermore, libiptcdata which we're using has been dead since 2009. IPTC is being phased out in favor of XMP. I believe both libexiv2 and ExifTool handle IPTC and XMP, solving that problem as well.

References:
http://owl.phy.queensu.ca/~phil/cpp_exiftool/
http://www.exiv2.org/getting-started.html#lib
#3581 (comment)
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