label: add elm.swallow.background part to all of groups
label: add elm.swallow.background part to all of groups

Summary:
elm.swallow.background part will be a default part for
widget's background image.

Test Plan: N/A

Reviewers: woohyun, cedric, kimcinoo

Differential Revision: https://phab.enlightenment.org/D3675

tasn/elementary,tasn/elementary,tasn/elementary,tasn/elementary,tasn/elementary