elementary: add internal Efl_Ui_Model_Exact.
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.
Reviewed-by: SangHyeon Jade Lee <firstname.lastname@example.org>
Differential Revision: https://phab.enlightenment.org/D7660