AVWriterPluginFactory: get supported extensions from avTranscoder

* Call getOutputExtensions function from avTranscoder to get supported
extensions of the plugin.
AVReaderPluginFactory: fit to avTranscoder

Update addOptionsToGroup: parameter rej_flags is not needed by
avTranscoder anymore.
AVWriterPluginFactory: use OptionLoader from avTranscoder

* Create addOptionsToGroup function to manage AVOption (ffmpeg options)
instead of call functions in libav wrapper.
AVPluginFactory Reader & Writer: refactoring

* Create common/util.hpp and define addOptionsToGroup functions, which
take a list of options, create OFX params from them, and set a parent
for new params.
* Suppress addOptionsToGroup function in AVReaderPluginFactory.
* Suppress addOptionsToGroup function in AVWriterPluginFactory.
* AVWriterPluginFactory: update #define variable (same model as
AVReaderPluginFactory).
AVPlugin: refactoring

* Split 'common/util' in two files: util.hpp and util.cpp.
AVWriterPluginFactory: use avTranscoder to fill OFX subgroup params

* Add options to videoDetailledGroup.
* Add options to audioDetailledGroup.
AVWriterPluginFactory: get list of format/codec names from avTranscoder

* Do not need a LibAVVideoWriter instance in describeInContext function:
   * get formats short names from avTranscoder.
   * get video codecs short names from avTranscoder.
   * get audio codecs short names from avTranscoder.