elementary: add internal Efl_Ui_Model_Average.
This model enable View that require to compute the size of their item
to rely on its logic to store all items size independently. It has the
same interface as the Homogeneous and should be exchangeable for a View.
It provide an average total size at all time. It use Efl.Ui.Model_Exact
to do most of the work and is a good example on how to alter a Model
Reviewed-by: SangHyeon Jade Lee <email@example.com>
Differential Revision: https://phab.enlightenment.org/D7661