WinGui: Implement hb_qsv_available in the UI so that the encoder isn't shown when the build hasn't been built with quicksync support or the system doesn't support it.

git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@5742 b64f7644-9d1e-0410-96f1-a4d463321fa5
WinGui: Fix a small logic bug in the video encoder dropdown with  av_mkv

git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@5743 b64f7644-9d1e-0410-96f1-a4d463321fa5