ecore: enforce proper lifecycle for all Efl.Composite_Model children.
This make sure that the object returned by children_slice_get are properly
destroyed when the refcount drop to only the parent holding a reference on
it. This make it clear that the user of the api can rely on efl_ref/efl_unref
to actually manage its use of the returned object.
Reviewers: felipealmeida, segfaultxavi, SanghyeonLee, zmike, bu5hm4n
Reviewed By: segfaultxavi, zmike
Maniphest Tasks: T7528
Differential Revision: https://phab.enlightenment.org/D7864