MacGui: Initial implementation of capping avialable bitrates depending on audio track...

MacGui: Initial implementation of capping avialable bitrates depending on audio track mixdown
- added AudioTrackMixdownChanged method to controller.mm which evaluates other selections and adjusts available bitrates to choose from
- currently just caps bitrates at 160 for anything other than 6 channel discrete
- 6 channel discrete in either audio track mixdown enables bitrates up to 384.
- Will likely need to add other conditions as well

git-svn-id: svn://localhost/HandBrake/trunk@533 b64f7644-9d1e-0410-96f1-a4d463321fa5