FFMPEG Plugin: Automatic detection of audio decoding parameters.

- Kudos to Marcus Overhagen for laying out the general idea of automatic
  detection by sharing some of his dvb code examples with me.
- Automatically detect the audio frame rate, channel count and sample format.
- Share audio sample format conversion code between AVFormatReader and
  AVCodecDecoder.
- Tested with several video and audio files via MediaPlayer.
- Tested also with test case mp3_decoder_test -after- removing the hard coded
  audio decoding parameters. Although the test shows that auto detection is
  working (via stepping through the auto detection code path) the complete test
  is still failing, due to missing implementation of incomplete audio frame
  decoding.
- Add and update the documentation accordingly.