MacGui: srt subtitle support initial implementation

MacGui: srt subtitle support initial implementation
- Allows adding an external srt sub file as a subtitle source.
-- Adds fields for srt language, char code and offset in ms.
- Known issues: adding the srt file to the sources list also adds a new subtitle track for that source, however to "set" it you have to click on it.
- srt subtitles do not seem to work in preview even if set to default.

git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@2699 b64f7644-9d1e-0410-96f1-a4d463321fa5