eo: Add API to allow deletion by unref

eo: Add API to allow deletion by unref

This allows deleting an object by simply calling efl_unref() on it, even
if there is a parent. Normally the parent is in charge, and you can
request deletion by calling efl_del() or efl_parent_set(NULL).

But in some rare cases, you want to give ownership of an object (@owned)
and still give a parent to that object. efl_unref() should be used (and
would be used by bindings when going out of scope or on garbage
collection), which would then print an error message. This API allows
the specific behaviour.

@feature