AVReaderPluginFactory: use OptionLoader from avTranscoder

* Create addOptionsToGroup function to manage AVOption (ffmpeg options)
instead of call functions in libav wrapper.
* Still dependency to ffmpeg (codec and format) when get supported
extensions and allocate a format and a codec.
AVReaderPluginFactory: add options to Audio and Metadata group parameters
AVReaderPluginFactory: suppress allocation of avFormat and avContext

* avFormat and avContext are manage by the OptionLoader of avTranscoder.
AVReaderPluginFactory: get supported extensions from avTranscoder

* Call getInputExtensions function from avTranscoder to get supported
extensions of the plugin.
* AVReader: no more dependencies to ffmpeg / libav !