which unlike HSL Lightness is actually physically meaningful and
also generally speaking much more useful than HSL Lightness.
Change "Lightness" to "Lightness (HSL)" to make it clear that
the "Lightness" in the Colors/Desaturate/Desaturate menu is not the
same as "Lightness" in LAB/LCH.
For completeness add the option to desaturate to "Value (HSV)".
Add links in app/operations/gimpoperationdesaturate.c
to the Wikipedia article with definitions of L/I/V in HSL/HSI/HSV.
Add GIMP_DESATURATE_LUMINANCE to enum GimpDesaturateMode and rename
GIMP_DESATURATE_LUMINOSITY to GIMP_DESATURATE_LUMA.
Keep GIMP_DESATURATE_LUMINOSITY as deprecated compat value and add it
to the script-fo and pygimp compat constants.
Change GimpOperationDesaturate to process GIMP_DESATURATE_LUMINANCE
with linear "RGBA float".
