efl: major rewrite of efl_part.
The interface efl_part_get should not be directly called from C, but the efl_part
wrapper should. It rely on efl_noref to properly destroy the object. Binding can
control the lifecycle of the reference the way they want by either calling the
wrapper or efl_part_get directly. It also means that the ugly ___efl_auto_unref_set
doesn't need to be exposed outside of EFL anymore.
Differential Revision: https://phab.enlightenment.org/D6098