AudioVideo: fit to new avTranscoder API

* DataStreamDesc renamed to DataStream.
AVWriter: add OFX parameters to transcode audio subStreams

* paramAudioAllChannels: checkbox to decide to transcode all channels of
the selected input stream.
* paramAudioChannelIndex: select a specific channel of the selected
input stream.
* Warning: these OFX parameters trigger nothing.
AVWriterPlugin: refactoring display of OFX params
AVWriterFactory: add hint to some OFX params

* Add a hint to audioStreamIndexParam.
* Add a hint to audioCodecPresetParam.
* Set the hint of audioCopyStreamParam.
* Update comment.
AVWriterPlugin: use audio subStream to transcode

* subStream = -1: default value, transcode the audio stream with all
channels.
* subStream = x: transcode the audio stream with the channel x.
AVWriterPlugin: fix audio transcode of all streams

* When inputStreamIndex = -1, transcode all audio streams of the input
audio file.
AVWriter: add OFX group param to contains custom format parameters

Keep the symmetry between format, video, and audio tabs.
AVWriterFactory: video / audio groups params are closed by default

The plugin is easier to use like this.
AVWriterPlugin: minor refactoring

* Reorder some functions.
* Add space.
* Move definition of local variable in initAudio.
AVWriterPlugin: no dummy audio stream if a file path is indicated

Easier to use the plugin with the command line (Sam).
AVWriterPlugin: no rewrap audio stream if an audio preset is indicated

Easier to use the plugin with the command line (Sam).
AVWriterPlugin: no transcode all channels of audio stream if a channel index is indicated

Easier to use the plugin with the command line (Sam).