list: Refactor direction key event handling routine in list.
list: Refactor direction key event handling routine in list.

- Added internal function _elm_list_elm_widget_event_direction.
- Simplified cascaded if statements.
- Note: focus_on_selection feature is still broken.

rvandegrift/elementary,tasn/elementary,FlorentRevest/Elementary,tasn/elementary,tasn/elementary,FlorentRevest/Elementary,FlorentRevest/Elementary,rvandegrift/elementary,tasn/elementary,rvandegrift/elementary,tasn/elementary,rvandegrift/elementary,FlorentRevest/Elementary