eo: add the ability to get the size of object of a certain class.
Their was two different way to implement this, either like this with
a simple function that work on Efl_Class, or by a function on
Efl.Object. I leaned on the first one, but I could easily be convinced
it should be done on Efl.Object actually.
Reviewed-by: Xavi Artigas <firstname.lastname@example.org>
Differential Revision: https://phab.enlightenment.org/D7441