edje_entry: Make a result of retrieve_surrounding_cb secure

edje_entry: Make a result of retrieve_surrounding_cb secure

Summary:
Regardless of the password mode of Entry,
_edje_entry_imf_retrieve_surrounding_cb alwalys passes plain_text
to any callers who register that callback.

This commit replace plain text with '*'
because current behavior could be a security hole in some case.

Reviewers: woohyun, id213sin, jihoon

Reviewed By: jihoon

Subscribers: cedric, jsuya, z-wony, jpeg

Differential Revision: https://phab.enlightenment.org/D4238