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".