FFMPEG Plugin: Implement flushing audio frames from decoder.

- Audio frames still contained in the video decoder are now flushed when there
  are no encoded data chunks left (signaled by B_LAST_BUFFER_ERROR). Should
  work in theory, I still need to get  mp3_decoder_test to run to really test
  it.

- Refactor code to support flushing -and- keep the readability at a sane level.

- Add documentation for new methods accordingly.
FFMPEG Plugin: Fix some white space typos.

- No functional change intended.
dvb.media_addon: Fix coding style.

- Add myself to copyright header.
- No functional change intended.
dvb.media_addon: Fix start time calculations.

- Both fixes don't seem to have a big impact for a/v sync, but at least we are
  more correct now and avoid confusing new developers with magic numbers (why
  add 13, neither the MPEG-2 standard nor any math rules I am aware off suggest
  this number).