command: export more information under track-list

Export a number of container fields, which may or may not be useful in
some scenarios. They are explicitly marked as originating from the
demuxer, in order to make it explicit that they might be unreliable.

I'd actually like to remove all other cases where container information
is exported, but those numerous cases are going to be somewhat hard to
deprecate.

Also, not directly related, export the description of the currently
active decoder. (This has been requested before.)

DOCS/interface-changes.rst

@@ -19,6 +19,9 @@ Interface changes
 
 ::
 
+ --- mpv 0.17.0 ---
+    - deprecate "track-list/N/audio-channels" property (use
+      "track-list/N/demux-channel-count" instead)
  --- mpv 0.16.0 ---
     - change --audio-channels default to stereo (use --audio-channels=auto to
       get the old default)