MacGui: Live Preview extend the live preview maximum duration to 240 seconds ( 4 min.) from the previous 120 seconds ( 2 minutes).

- Uses 15 second incremements as previous.

git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@5258 b64f7644-9d1e-0410-96f1-a4d463321fa5