elm_focus: implement elm_object_focus_next_item_set / get

you can use this now to let the focus move to the widget container of
the passed item.

I know this patch contains a whitespace change, but i have to get out
this whitespace each & every time i am editing the file - which is
annoying. So remove it once, which makes further work easier.

fixes T6183.

Reviewed-by: YeongJong Lee <yj34.lee@samsung.com>
Differential Revision: https://phab.enlightenment.org/D7408