AudioVideo writer: rename parameter a_nbStream to a_nbInput
AudioVideo writer: fix audio mainPreset when rewrap

Adding "rewrap" choice to a_mainPreset parameter involves us to update
the offset when looking for an existing audio preset.
AudioVideo writer: clean presets management

* Add comments.
* Compare distance between iterators (begin and search iterator) with
size of structure.
AudioVideo writer: get sar from FFmpeg when create custom video preset

Get sar FFmpeg parameter from
_paramVideoCustom.getCorrespondingProfile().
AudioVideo common: clean include

Remove include of boost/lexical_cast.