Genlist: On focus_on_selection mode, the focus should set on item content objects...

Genlist: On focus_on_selection mode, the focus should set on item content objects of item type ELM_GENLIST_ITEM_TREE.

Summary:
On focus_on_selection set, the focus set on genlist item of type ELM_GENLIST_ITEM_TREE rather than
         on item content objects.

Test Plan: elementary_test -to "genlist focus"

Reviewers: seoz, eagleeye, SanghyeonLee, raster

Reviewed By: raster

CC: seoz
Differential Revision: https://phab.enlightenment.org/D899