#3 Move Forced-only selection out of options into stream sel.

[Mario Rohkrämer]

Because the requirement to process forced subtitles only or all subtitles of a stream may change for each DVD, it is probably useful to implement easier access to this option than hiding it far away in the application options; it is quite closely related to each workflow between opening a DVD source and starting the OCR process instead. I believe it should belong to the selection of the subtitle stream.