gengrid: Fix mirroring issue about key event

gengrid: Fix mirroring issue about key event

Summary:
Originally, key "Left" is mapped elm_gengrid_item_prev_get. But when gengrid is mirrored, key "Left" should remap elm_gengrid_item_next_get.
So I add more condition for checking mirroring.

1. reverse Left and Right key event
2. fix _elm_gengrid_item_edge_check logic
3. fix _item_show_region logic

Test Plan: elementary_test -> gengrid

Reviewers: SanghyeonLee

Subscribers: seoz

Differential Revision: https://phab.enlightenment.org/D1381

Conflicts:
	src/lib/elm_gengrid.c