Update French translation of usermanual
Proof-read till "Velvia and Channel mixer" modules.

Translate in all the manual
  lightness = clarté (of a reflective object)
  brightness = luminosité (of a light source object)
  luminance = clarté

To be consistent with French darktable's GUI I translate "core options" as "fonctionnement" in all the usermanual

A "make update-usermanual-fr" has been necessary to fix the errors due to incorrect uses of hyphens.
First use of poedit 1.5.4 which manages better the lines
Some error corrections in the header of the file fr.po
Merge pull request #667 from VictorLamoine/master

Update French translation of usermanual
exif: disable draconic dmc-gh2 workaround (#10074)
Redo error checking logic for stat()

Reimplements 7cca3bc9326ac3a56d1c70af29fc81939ca37b70