dict: add av_dict_count()

This adds a function to retrieve the number of entries in a
dictionary and updates the places directly accessing what should
be an opaque struct to use this new function instead.

Signed-off-by: Mans Rullgard <mans@mansr.com>

doc/APIchanges

@@ -13,6 +13,9 @@ libavutil:     2011-04-18
 
 API changes, most recent first:
 
+2012-08-08 - xxxxxxx - lavu 51.38 - dict.h
+  Add av_dict_count().
+
 2012-08-xx - xxxxxxx - lavc 54.25 - avcodec.h
   Rename CodecID to AVCodecID and all CODEC_ID_* to AV_CODEC_ID_*.