"Render Audio" cleanup:
* Removed audio-only options from ffmpeg render settings (added some versionning code too)!
* Moved the Mixdon button from the Scene->Audio pannel to the Render->Render panel.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk@61190 954f8c5b-7b00-dc11-b283-0030488c597c