MacGui: Store live previews in a sub directory in "~/Library/Application Support...

MacGui: Store live previews in a sub directory in "~/Library/Application Support/HandBrake/Previews" named by pidnum.
- Allows multi-instances to encode live previews without overwriting the live preview for any other instance (since previously we assumed single instance so there was just one live preview file for each container.
- Clean up of the previews directory when a single instance is laucnhed to make sure we do not build up a pile of old previews.
- Removed old code for live previews which handled the deprecated avi and ogm containers.

git-svn-id: svn://localhost/HandBrake/trunk@3472 b64f7644-9d1e-0410-96f1-a4d463321fa5