datetime: add color classes and text classes.

datetime: add color classes and text classes.

Summary:
The following new color classes and text classes
for datetime widget are added.

"datetime_bg", "Datetime Background"
"datetime_separator_text", "Datetime Separator Text"
"datetime_separator_text_disabled", "Datetime Separator Disabled Text"

@feature

Reviewers: raster, cedric, kimcinoo, Hermet, woohyun

Subscribers: SanghyeonLee, eagleeye, id213sin

Differential Revision: https://phab.enlightenment.org/D3485

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