gst/gstbin.c: Cleanup the get_state() function a little, make sure it iterates the same set of elements.

Original commit message from CVS:
* gst/gstbin.c: (gst_bin_get_state), (gst_bin_iterate_state_order),
(gst_bin_change_state):
Cleanup the get_state() function a little, make sure it
iterates the same set of elements.
Added stub iterate_state_order().