libcolord: Add cd_it8_set_enable_created()

This allows us to supress the date and time in the IT8 file which allows us to
string compare it to a known reference in the self test program.
libcolord: Always write C-locale floating point values in IT8 files

LCMS uses 'sprintf(.10%g)' to convert floating point values to strings.
If LC_NUMERIC is set to something like 'nl_BE' then values such as 1.234 are
printed as '1,234'. This is against the CGATS standard and Argyll correctly
refuses to parse the file.

Wrap each call to get and set the floating point values with the safe GLib
functions of g_ascii_strtod() and g_ascii_dtostr().