efl_gfx_color_class: make this a mixin and move color_class_code here
Summary:
color_class_code seems to only have been added to efl_gfx_color in order to
share the hex conversion code, but this is trivial to do when it's in another
class and makes the api more sensible
ref T7559
Reviewers: segfaultxavi, bu5hm4n, cedric
Reviewed By: segfaultxavi
Subscribers: reviewers, committers
Tags: efl: api
Maniphest Tasks: T7559
Differential Revision: https://phab.enlightenment.org/D7898