AudioVideo: refactoring

Call addOptionsToGroup( ..., OptionArray, ...) in addOptionsToGroup(
..., OptionMap, ...).
AudioVideo: refactoring

Rename codecName parameter to subGroupName (this can be a codec name, or
something else).
AVWriter: refactoring

Call fetchCustomParams( ..., OptionArray, ...) in fetchCustomParams(
..., OptionMap, ...).
AVWriter: fetch custom video and audio codec params

Still not adding them to the video or audio profile.
AVWriter: update name of OFX params from FFMPEG

* For all OFX params created from AVOption (FFMPEG), their name is, in
order:
     * prefix (f_, v_, a_, or m_).
     * "g_" if it's a group.
     * subGroupName if indicated (the codec name for example).
     * "_" if subGroupName if is indicated.
     * name of the param.
     * if it's child of a group, the unit (name of his group).
     * if it's child of a group, "flags_" (because all group names are
not composed by "flag" and we need this information).
     * if it's child of a group, name of the child.
* Update getOptionNameWithoutPrefix function to fit to this new pattern.
AudioVideo: refactoring

* The structure which stores the list of option and their value for
create a custom profile, is now a map. The key / value wiil be useful
with the "flags" FFMPEG params (we need to concatenate several values
for a unique key).
AudioVideo: fix OFX String params from FFMPEG

For OFX String params from FFMPEG Option, don't add it if the value is
an empty string (error in FFMPEG).
AVWriter: manage OFX flags params from FFMPEG

* For this kind of OFX params from FFMPEG Option, we need to concatenate
all values in a single option with the name of the flag.
* Add getOptionFlagName function.
AVWriter: use custom video and audio params to get custom profiles

* Warning: problem with the default value of 'sample rate' and 'channel'
for an audio stream in FFMPEG. Temporary solution: overwrite these
values before setting the profile of audio stream.
AVWriter: add OFX param for audio sample format

Same idea as the pixel format for a video stream.
AVWriter: fetch custom format params

Still not adding them to a custom format profile (not implemented yet in
avTranscoder).
AudioVideo: refactoring

* common/util: use static const variables to get name of prefix (group,
flag...).
* AVWriter: update comments.
AVWriter: clean fix of custom audio profile

* This fix checks some values of the profiles, and overwrite them if
incorrect.
* Now this is managed in avTranscoder, and raise an exception if
incorrect values.