FFMPEG Plugin: Print debug output for audio frames, too

- Also change what is printed for video frames. Currently both
  debug_fframe_[audio|video] are used in AVCodecDecoder only and thus are
  streamlined for their usage there. For example we print the AVFrame.pkt_dts
  field instead of the AVFrame.pkt field because the later one is never touched
  by AVCodecDecoders usage of the FFMPEG library.
  Note: AVFrame.pkt being never touched means that it always contains the value
  AV_NOPTS_VALUE making it less useful for debug purposes.