ext/ffmpeg/: Add simple query functions. Seeking is only cosmetic, it's not actually filled in yet (in ffmpegdec).

Original commit message from CVS:
* ext/ffmpeg/gstffmpegdec.c: (gst_ffmpegdec_init),
(gst_ffmpegdec_query), (gst_ffmpegdec_event), (gst_ffmpegdec_open),
(gst_ffmpegdec_negotiate), (gst_ffmpegdec_chain):
* ext/ffmpeg/gstffmpegdemux.c: (gst_ffmpegdemux_init),
(gst_ffmpegdemux_close), (gst_ffmpegdemux_src_query),
(gst_ffmpegdemux_loop):
Add simple query functions. Seeking is only cosmetic, it's not
actually filled in yet (in ffmpegdec).