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