edje: add a helper for recursive searching in hash

edje: add a helper for recursive searching in hash

Summary:
If color class of an edje part is defined as "aaa/bbb/ccc/ddd",
edje will search for color class by the following sequence.
   "aaa/bbb/ccc/ddd"
   "aaa/bbb/ddd"
   "aaa/ddd"
   "ddd"
So, without additional lookup table, edje classes (color, text, size)
can have the functionality like inheritance.

Reviewers: jpeg, raster, cedric

Subscribers: cedric

Differential Revision: https://phab.enlightenment.org/D4127

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