gengrid: add unhighlight callback

gengrid: add unhighlight callback

Summary:
item_unselect check it->select status, so when item highlighted but not selected,
item highlight is remaining even after unselect_cb is called.

And item_select and item_highlight are separated, but item_unselect and item_unhighlight are mixed.
so divide them, too.

This patch will solve upon problem.

Reviewers: seoz, woohyun, Hermet, CHAN, raster, SanghyeonLee, cedric

Reviewed By: cedric

Differential Revision: https://phab.enlightenment.org/D2653

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