Stage 1 Soft Subtitle Support - Allow multiple subtitle tracks to be selected, and...

Stage 1 Soft Subtitle Support - Allow multiple subtitle tracks to be selected, and mark them for Render or Pass Through. No functional difference from the UIs, however in theory they could render multiple tracks - would be a dogs breakfast on screen though, and is untested. Have tested all normal operations from the UIs, including scanning.

git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@2373 b64f7644-9d1e-0410-96f1-a4d463321fa5