ext/mad/gstid3tag.c: deprecate id3tag element and replace with id3demux/id3mux.

ext/mad/gstid3tag.c: deprecate id3tag element and replace with id3demux/id3mux.

Original commit message from CVS:
* ext/mad/gstid3tag.c: (gst_id3_tag_get_type),
(gst_id3_tag_class_init), (gst_id3_tag_get_caps),
(gst_id3_tag_add_src_pad), (gst_id3_tag_init),
(gst_id3_tag_set_property), (gst_id3_tag_do_caps_nego),
(gst_id3_tag_src_link), (gst_id3_tag_chain),
(gst_id3_tag_change_state), (plugin_init):
deprecate id3tag element and replace with id3demux/id3mux.
great side effect: this ugly file is now even uglier, yay!
* ext/mad/gstmad.h:
remove non-available function
update for new get_type