MacGui: Fix and issue for auto adding the .m4v file extension for the aac + ac3 encod...

MacGui: Fix and issue for auto adding the .m4v file extension for the aac + ac3 encoder for mp4 where reselecting mp4 format would reset the extension back to .mp4 and not .m4v
- Moves this to a new method called codecsPopUpChanged: so it is easier to call form multiple methods

git-svn-id: svn://localhost/HandBrake/trunk@1309 b64f7644-9d1e-0410-96f1-a4d463321fa5