efl: add a Efl.Model_Provider that every widget will look up for in their parent...

efl: add a Efl.Model_Provider that every widget will look up for in their parent tree.

This is done to simplify code as you only need to set the model on the
provider and all the widget that are using it as a provider will automatically be
updated. The child will find a provider during at the time the first property binding
is set on the widget by checking if the parent have an Efl.Model_Provider set. It is
not necessary to set a model to have a valid lookup on a Efl.Model_Provider. To disable
a widget lookup, you can just force set a model on it (even NULL) and it will disable
the lookup.

Reviewed-by: Marcel Hollerbach <mail@marcel-hollerbach.de>
Differential Revision: https://phab.enlightenment.org/D9290