LinGui: add CFR option for "Same as source" framerate.

LinGui: add CFR option for "Same as source" framerate.
Since there seem to be a lot of players that do not properly support VFR,
add a CFR option that can be used with "Same as source".  The framerate to
use comes from the title and the cfr flag is set in the job.

git-svn-id: svn://localhost/HandBrake/trunk@3770 b64f7644-9d1e-0410-96f1-a4d463321fa5