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