WinGui: Enhancements to the preview window and scanning option.s
- Allow up to 60 preview images and expand the video preview selection up to 240 seconds.
- Allow a preview to be generated when the queue is already running.
- Grey out the preview Play button when it's encoding.

git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@5263 b64f7644-9d1e-0410-96f1-a4d463321fa5