ui/flip: fix efl_pack usage

Objects packed would end up invisible as they were not marked as
"_elm_leaveme". I also added some list safety code but I think the
entire pack API on flip is very much untested and unstable.

@fix

Reviewed-by: Cedric BAIL <cedric.bail@free.fr>
Reviewed-by: Xavi Artigas <xavierartigas@yahoo.es>
Differential Revision: https://phab.enlightenment.org/D7367