FFMPEG plugin: Add support for profiling audio decoding.

Adds variables for measuring the decoding performance of the audio
decoder. This could have been solved by reusing the existing video
profiling variables, but I prefer telling clearly what a variable
is used for. This telling happens by means of prefixing the
profiling variables with "audio" respectively with "video".

Remove profiling code for printing the current profiling data when
the ffmpeg plugin is destroyed (AVCodecDecoder::~AVCodecDecoder()).
I see only little value in keepingthis code around as you really
wanne have the profiling information during the decoding and not
at the end. And keeping the profiling code around would have mean
seperate code for audio and video decoding when sticking with the
variable naming scheme I have chosen (see comment above) which
would be a wasted effort in contrast to the gain (having the
profiling information of the last decoding run printed out on
destruction of the ffmpeg plugin).