FFMPEG Plugin: Refactor out chunk loading in audio path.

- Main reasons are to increase readability of audio path and to demonstrate
  that chunk loading in audio and video path is the same code that can be
  focused in one method (instead of two at the moment). Added a TODO for
  collapsing both methods into one and the conditions that must hold true to
  do so (just in case I'll be hitted by a bus and someone else has to proceed).
  Collapsing is scheduled for a later commit.

- Added documentation for the new method accordingly.

- Make use of full line length in comments of
  _LoadNextVideoChunkIfNeededAndAssignStartTime().

- No functional change intended.