eina_model: add xref/xunref, xrefs_get and models_usage_dump!

eina_model: add xref/xunref, xrefs_get and models_usage_dump!

Let's try to help debug by allowing extended reference management that
takes in account an identifier. This identifier is accounted on xref
and xunref and must match.

xrefs_get will return the list of such references, for debugging purposes.

eina_models_list_get() was added to return all live models, just
tracked when EINA_MODEL_DEBUG is enabled.

eina_models_usage_dump() was added and use the same infrastructure as
eina_models_list_get() and eina_model_xrefs_get() to aid debugging :-)

git-svn-id: http://svn.enlightenment.org/svn/e/trunk/eina@67821 7cbeb6ba-43b4-40fd-8cce-4c39aea84d33