MacGUI: Replace deprecated NSBeginCriticalAlertSheet / NSRunAlertPanel / NSRunCriticalAlertPanel with NSAlert. Patch https://reviews.handbrake.fr/r/711/ by Alexander Zautke with a few modifications by me.

git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@6294 b64f7644-9d1e-0410-96f1-a4d463321fa5