WinGui:  Fix a styling issue on the preview window.  Disable libhb encode for the upcoming release so that users don't inadvertently turn it on.  Made the new preview window an optional setting, off by default as it's a Alpha feature. (See Preferences) 

git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@6256 b64f7644-9d1e-0410-96f1-a4d463321fa5