panel: Remove error message from elm_panel_add API and _theme_apply func.

panel: Remove error message from elm_panel_add API and _theme_apply func.

Summary:
elm_layout_content_set API calls _elm_panel_elm_container_content_set func.
The panel widget checks the given "parts" string is whether "elm.swallow.event" or not.
To avoid error message and set the given object to the "elm.swallow.event" internally,
the panel widget need to call *_content_set for "elm.swallow.event" part using eo_do_super.
@fix

Test Plan: elementary_test -> panel

Reviewers: seoz, woohyun, Hermet

Reviewed By: Hermet

Differential Revision: https://phab.enlightenment.org/D1949