MacGui: In Preferences, Instead of the old "Use iTunes / iPod friendly file extension...

MacGui: In Preferences, Instead of the old "Use iTunes / iPod friendly file extension for mp4" checkbox we allow three choices:
- Auto (Default): will use .mp4 unless any of the conditions are met for Apple devices to use .m4v such as AC3 audio or chapters, etc.
- .m4v: will set the extension to .m4v regardless of settings.
- .mp4: will set the extension to .mp4 regardless of settings.
Basically anything other than Auto forces either the .m4v or .mp4 file extension. Again "Auto" is default.
Note: This deprecates the old preferences plist key "DefaultMpegName" which was just a bool and adds "DefaultMpegExtension" which is a string. As a result users that had the old "Use iPod/iTunes friendly (.m4v) file extension for MP4" checkbox checked will need to select ".m4v" in the new preferences dialogue.

git-svn-id: svn://localhost/HandBrake/trunk@3758 b64f7644-9d1e-0410-96f1-a4d463321fa5