AVWriter: fix OFX param names of AudioSubGroups

* Update their name to use them in script:
     * audioSubGroup_0, audioSubGroup_1...
     * audioFilePath_0, audioFilePath_1...
     * audioStreamId_0, audioStreamId_1...
     * audioPreset_0, audioPreset_1...
AVWriter: refactoring

* Rename kParamVideoPreset to kParamMainVideoPreset.
* Rename kParamAudioPreset to kParamMainAudioPreset.
* Add kParamAudioPreset to set the name of audioCodecPresetParams in the
audioSubGroups list.
* Also rename the OFX params concerned in the Plugin.
AVWriterFactory: refactoring
AVWriter: add audioCustomGroupParam

* This param contains the audioCodecParam (which is moved before the
list of audioSubGroups).
* The group is secret and disable if a preset is set in the list of
presets. It appears only if 'custom' is set in the list of presets.