efl_ui : add efl.extra part as layout swallow part list.

Summary:
efl.extra part is extra part of the layout swallow,
which can be generally used on overall layout object.
To be supported layout part infrastructures,
add efl.extra on layout_swallow_part[] list.

Depends on D9233

Reviewers: cedric, bu5hm4n, zmike, segfaultxavi

Reviewed By: bu5hm4n

Subscribers: #reviewers, #committers

Tags: #efl

Differential Revision: https://phab.enlightenment.org/D9316

src/lib/elementary/efl_ui_layout.c

@@ -67,6 +67,7 @@ static const char *_efl_ui_layout_swallow_parts[] = {
    "efl.icon",
    "efl.end",
    "efl.background",
+   "efl.extra",
    NULL
 };