AVWriter: refactoring

* AVWriterPlugin:
     * update comments.
     * AVProcessParams structure: renamed _filepath to _outputFilePath.
* AVWriterPluginFactory:
     * fix label of videoCodecPixelFmt OFX param.
     * use avTranscoder to get the list of supported pixel formats
depending on the videoCodec (and add the list as options to
videoCodecPixelFmt OFX param).
* AVWriterProcess:
     * update comments.
AVWriter: add audio transcoding

* Resume: The plugin can get an input audio file, select a stream id,
and when encode video also transcode audio to save image AND audio in
the same output file.

* AVWriterFactory:
     * add 3 OFX params: number of stream, audio file path, and audio
stream id.
* AVWriterPlugin:
     * AVProcessParams structure: add _inputAudioFilePath.
     * add avTranscoder objects to get an input file, extract a stream,
read frame, convert audio and encode audio.
     * separate ensureWriterIsInit function: ensureWriterIsInit and
ensureAudioIsInit (because the plugin can encode video whithout audio).
* AVWriterProcess:
     * transcode audio stream selected if an input audio file is set.

* Warnings:
     * can transcode only one audio stream.
     * can get only one audio file.