edje_entry: Support multibyte on result of retrieve_surrounding_cb

edje_entry: Support multibyte on result of retrieve_surrounding_cb

Summary:
to make a result of retrieve_surrounding_cb secure,
callback result string will be replaced by star('*') when entry is password mode.

however, if plain_text was multibyte,
too many stars are displayed than it's actual length.

Reviewers: jihoon, id213sin, woohyun, herdsman

Subscribers: herdsman, cedric, jpeg

Differential Revision: https://phab.enlightenment.org/D4427

Change-Id: Id321bf7c7db987a3795c4f33d80554f36e6bb06b
Signed-off-by: Wonki Kim <wonki_.kim@samsung.com>