focus: Added optional focus feature - item focus on selection.

focus: Added optional focus feature - item focus on selection.

Summary:
Currenly focus and selection happens together. This patch makes item focus
on selection optional by configuration and API.

- configuration: "item_focus_on_selection".
- API: elm_config_item_focus_on_selection_get/set.
- Implemented item_focus_on_selection feature in elm list.
@feature

Test Plan: elementary_test->List focus

Reviewers: raster, seoz, woohyun

CC: seoz
Differential Revision: https://phab.enlightenment.org/D676