eina_model: change the way extensions are done.

eina_model: change the way extensions are done.

Thanks to Tasn review it was noticed that subclasses using "const void *value"
with sub-structure would have to resolve their methods manually, quite bad.

Then the extension will happen with structures inside structures, as
previously used in Evas_Smart_Class. To allow the system to resolve
the methods and keep it, it is now specificed the type_size.

A new function is added to help extensions:
eina_model_type_method_resolve(), together with few helper macros.

To allow Eina_Model_Type to be augmented with more methods without
breaking the ABI, 4 slots are left there. We expect we got it right
from beginning and these will never be used, but we can never be sure
;-)

Last but not least, similar changes were done to Eina_Model_Interface
for consistency matters.

SVN revision: 67635