player: change video-bitrate and audio-bitrate properties

Remove the old implementation for these properties. It was never very
good, often returned very innaccurate values or just 0, and was static
even if the source was variable bitrate. Replace it with the
implementation of "packet-video-bitrate". Mark the "packet-..."
properties as deprecated. (The effective difference is different
formatting, and returning the raw value in bits instead of kilobits.)

Also extend the documentation a little.

It appears at least some decoders (sipr?) need the
AVCodecContext.bit_rate field set, so this one is still passed through.

audio/decode/ad_lavc.c

@@ -144,9 +144,6 @@ static int init(struct dec_audio *da, const char *decoder)
         return 0;
     }
 
-    if (lavc_context->bit_rate != 0)
-        da->bitrate = lavc_context->bit_rate;
-
     return 1;
 }