edje: fix lifecycle of swallowed children

edje: fix lifecycle of swallowed children

Summary:
a swallowed child is per doc not deleted when the edje is deleted.
After the call to invalidate is finished all children will loose theire
parents and also be invalidated. However, in the efl_invalidate call,
the event EVAS_CALLBACK_DEL is issued, so this is the right spot. As in
legacy the swallows are in place when the EVAS_CALLBACK_DEL event is
issued.

Reviewers: cedric, raster, JackDanielZ, zmike

Subscribers: cedric, #committers, zmike

Tags: #efl

Differential Revision: https://phab.enlightenment.org/D6222

Reviewed-by: Cedric BAIL <cedric.bail@free.fr>