Iterate over 'struct varobj_item' instead of PyObject

Iterate over 'struct varobj_item' instead of PyObject

In previous patch, "saved_item" is still a PyOjbect and iteration is
still performed over PyObject.  This patch continues to decouple
iteration from python code, so it changes its type to "struct
varobj_item *", so that the iterator itself is independent of python.

 V2:
 - Call varobj_delete_iter in free_variable.
 - Fix changelog entries.
 - Use XNEW.

 V3:
 - Return NULL early in py_varobj_iter_next if gdb_python_initialized
   is false.

gdb:

2014-06-12  Pedro Alves  <pedro@codesourcery.com>
	    Yao Qi  <yao@codesourcery.com>

	* python/py-varobj.c (py_varobj_iter_next): Return NULL if
	gdb_python_initialized is false.  Move some code from varobj.c.
	* varobj-iter.h (struct varobj_item): Moved from varobj.c.
	* varobj.c: Move "varobj-iter.h" inclusion earlier.
	(struct varobj_item): Moved to varobj-iter.h".
	(varobj_clear_saved_item): New function.
	(update_dynamic_varobj_children): Move python-related code to
	py-varobj.c.
	(free_variable): Call varobj_clear_saved_item and
	varobj_iter_delete.