Embedded VobSub track: honor the per-subtitle forced flag when the index tells to do so.
External VobSub track: Fix the per-subtitle forced flag.
This flag was currently never honored because the track setting was mistakenly overwritten by the Subresync bar.
PGS subtitles: Prepare support for the per-subtitle forced flag.

Currently this feature is always turned off and can't be controlled by the user.
Split rendered DVB and PGS subtitles into two classes.

First step for per-subtitle forced flag control.
PGS subtitles: Add user control for forced subtitles.

Show two tracks: one with all subtitles and the other with only the forced subtitles.