AudioVideo: writer - remove unnecessary _videoStream attribute
AudioVideo: rename CustomParams to LibAVParams

* This class enable to get libav Options about format, video, and audio.
* struct to class.
* Update documentation of the class.
AudioVideo: refactore LibAVParams class

* Remove 3 typedef.
* Rename typedef OptionsForPreset to LibAVOptions.
* Rename getOptionsNameAndValue function to getLibAVOptions.
* Update _childsPerChoice attribute: key of the map is a pointer to an
OFX::ChoiceParam.
AudioVideo: fit to AvTranscoder API

Rename Profile to ProfileLoader.
AudioVideo writer: update comments
AudioVideo writer: refactore getProcessParams - metadatas
AudioVideo: update how to set libAVOption

Get the FFmpeg option, set its value, and return it to update the
corresponding OFX parameter.
AudioVideo writer: fix 'update Format/Video/Audio FromExistingProfile'
AudioVideo writer: refactore getProcessParams

Add getters which throw exception if format/codec are not found.
AudioVideo writer: add documentation
AudioVideo writer: set audio default preset to rewrap

The basic case for the user is to rewrap the audio files he wants.
AudioVideo: fix audio preset when it comes from main preset parameter
AudioVideo writer: clean AVWriterPluginFactory

Rename formatPreset to formatPresetParam (like other parameters, finish
the name by "Param").
AudioVideo writer: fix video encode

* FFmpeg has options which have no value possible, except "unknown"
("profile" option when encode video). But any option with this value
should not be set to the encoder if they are unknown...
* Ugly fix but I don't have other solution (and it prevents this problem
for any parameters in any FFmpeg versions...).
AudioVideo writer: fix audio encode when profile comes from main preset
AudioVideo writer: change nbStream audio parameter when add new filePath

* No need to set "a_nbStream" parameter in command line for adding new
audio inputs.