elementary: add internal Efl_Ui_Model_Size.
This model enable View that require to compute the size of their items
to rely on an interface to provide the properties they need to get the object
size. This is the base class for all the sizing logic of the new List/Grid View.
Reviewed-by: SangHyeon Jade Lee <firstname.lastname@example.org>
Differential Revision: https://phab.enlightenment.org/D7658