ecore: prevent duplication of children for Efl.Composite_Model and enable dummy object.

ecore: prevent duplication of children for Efl.Composite_Model and enable dummy object.

This patch allow Efl.Composite_Model to return always the same object for the same index.
This way, it make it easier for the Model to always be in sync even if there is multiple
user at any time. The support for dummy object allow the Composite_Model to host more
object than what the source model provide. This dummy model will only have the property
of the Composite_Model and none of the Source model ofcourse.

Reviewed-by: Vitor Sousa da Silva <vitorsousa@expertisesolutions.com.br>
Differential Revision: https://phab.enlightenment.org/D8045