[elc_popup] - Added focus support on popup-base's swallow parts.

[elc_popup] - Added focus support on popup-base's swallow parts.

Summary:
 Currently the focus was only on popup-content and popup-action area.
New API elm_layout_content_swallow_list_get() which returns the list of
swallow parts.

Test Plan: elementary_test -to popup. click on "subpopup + X button"

Reviewers: seoz, raster, woohyun

Reviewed By: raster

CC: nirajkr, aryarockstar, kashish
Differential Revision: https://phab.enlightenment.org/D375