Add a GType to GstIterator, update callsites and tests.

Add a GType to GstIterator, update callsites and tests.

Original commit message from CVS:
* check/gst/gstiterator.c: (GST_START_TEST):
* gst/gstbin.c: (gst_bin_iterate_elements),
(gst_bin_iterate_recurse), (gst_bin_iterate_sorted):
* gst/gstelement.c: (gst_element_iterate_pads):
* gst/gstformat.c: (gst_format_iterate_definitions):
* gst/gstiterator.c: (gst_iterator_init), (gst_iterator_new),
(gst_iterator_new_list), (gst_iterator_filter):
* gst/gstiterator.h:
* gst/gstquery.c: (gst_query_type_iterate_definitions):
Add a GType to GstIterator, update callsites and tests.