edje: introduce color_tree (color_class inheritance)

edje: introduce color_tree (color_class inheritance)

Summary:
The "color_tree" block contains a list of one or more "node" blocks.
A "node" block consists of its own color class name and the list of child
color classes. At runtime, parent color class will be referred instead,
if child color class is set to part but its color values are not defined.

Reviewers: raster, Jaehyun_Cho, jpeg, cedric

Reviewed By: cedric

Subscribers: cedric, kimcinoo

Differential Revision: https://phab.enlightenment.org/D3606

Signed-off-by: Cedric BAIL <cedric@osg.samsung.com>