MacGui: Fix issue from 1703 where once done doing at least one encode, all subsequent...

MacGui: Fix issue from 1703 where once done doing at least one encode, all subsequent jobs would require that you add to queue first, then hit start.
- Now, even after encoding a queue is finished, you can just hit "Start" to add the currently loaded job in the GUI to the queue and start encoding right away.

git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@1715 b64f7644-9d1e-0410-96f1-a4d463321fa5