MacGUI: Refactor HBPreviewController. Rename the actual class to HBPreviewController, and move the image and video creation code to a separate HBPreviewGenerator class. Update the preview duration list as Rodeo suggested.

git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@5874 b64f7644-9d1e-0410-96f1-a4d463321fa5