ext/ffmpeg/gstffmpegdemux.c: Rework the demuxer, implement all seeking stuff including seek in ready.

Original commit message from CVS:
* ext/ffmpeg/gstffmpegdemux.c: (gst_ffmpegdemux_base_init),
(gst_ffmpegdemux_class_init), (gst_ffmpegdemux_init),
(gst_ffmpegdemux_close), (gst_ffmpegdemux_push_event),
(gst_ffmpegdemux_set_flags), (gst_ffmpegdemux_is_eos),
(gst_ffmpegdemux_do_seek), (gst_ffmpegdemux_perform_seek),
(gst_ffmpegdemux_src_event), (gst_ffmpegdemux_send_event),
(gst_ffmpegdemux_src_query), (gst_ffmpegdemux_src_convert),
(gst_ffmpegdemux_get_stream), (my_safe_copy),
(gst_ffmpegdemux_read_tags), (gst_ffmpegdemux_open),
(gst_ffmpegdemux_loop), (gst_ffmpegdemux_sink_activate),
(gst_ffmpegdemux_sink_activate_pull),
(gst_ffmpegdemux_change_state), (gst_ffmpegdemux_register):
Rework the demuxer, implement all seeking stuff including
seek in ready.
* ext/ffmpeg/gstffmpegprotocol.c: (gst_ffmpegdata_peek),
(gst_ffmpegdata_close):
Handle some more cases.