MacGui: Clean up a few things w/x264optstring prefs and presets

MacGui: Clean up a few things w/x264optstring prefs and presets
- Change abr of AppleTV built in preset to 2500 kbps
- Note: as usual, after updating your copy, use "Update Built-In Presets" in the presets menu for changes to take effect.
- Fix so that closing picture window does not reset the preset chosen display to "Custom"
- Add a user preference to show or hide the chosen x264 option strings in the main gui window. Defaults to off.

git-svn-id: svn://localhost/HandBrake/trunk@525 b64f7644-9d1e-0410-96f1-a4d463321fa5