elementary: add internal Efl_Ui_Model_Homogeneous.
This model enable View that require to compute the size of their item
to rely on its logic to have all items of the same size. It is the
equivalent of the Homogeneous behavior of Genlist, except that now this
behavior can be customized outside of the View logic itself.
Differential Revision: https://phab.enlightenment.org/D7659