MacGui: fix issues with audio in presets

MacGui: fix issues with audio in presets
- Change audio track behavior so that presets only set the audio tracks it has stored, and removes any other previously selected audio tracks
- Prevents a scenario whereby selecting the ATV preset then the iPod preset results in two identical audio tracks for the iPod
- Fix an issue where certain mixdowns and bitrates contained in a preset may not be available depending on a source which resulted in no selection.

git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@1499 b64f7644-9d1e-0410-96f1-a4d463321fa5