Implement the exif extraction

[AngeloFrangione]

The exif fields are in the database, but nothing is populated during import time.
ressources:

• https://libexif.github.io/api/exif-loader_8h.html#a2b96cbffaf0f92340a960f887026eced

todo:

• create the libexif wrapper
• implement the wrapper functions at import time