gst/id3demux/: Use new typefind helper functions here as well, and do typefinding in pull-mode if upstream supports t...

Original commit message from CVS:
* gst/id3demux/Makefile.am:
* gst/id3demux/gstid3demux.c: (gst_id3demux_add_srcpad),
(gst_id3demux_chain), (gst_id3demux_sink_activate):
Use new typefind helper functions here as well, and
do typefinding in pull-mode if upstream supports that.