lavf/movenc: fix writing of some iTunes metadata tag

Fix metadata display under iTunes for files using pgap, hdvd, stick,
tvsn and tves tags.
lavf/movenc: support iTunes cover art

Cover art muxing is done by introducing the -cover_stream_index option
which takes an output stream index as argument.
The stream used for the cover art is not muxed as a track in the
resulting file.