ecore: enforce proper lifecycle for all Efl.Composite_Model children.

ecore: enforce proper lifecycle for all Efl.Composite_Model children.

Summary:
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

Subscribers: #reviewers, #committers

Tags: #efl

Maniphest Tasks: T7528

Differential Revision: https://phab.enlightenment.org/D7864