Initial support for forced subtitles (DVD & Blu-ray)

Forced subtitles are used in movies and series whenever a foreign/alien language is spoken to make the dialogue understandable for viewers who don't know this language. This type of subtitle is used a lot in series like Game of Thrones for the fictional Dothraki language and is relevant to understanding the whole plot.
On physical DVD and Blu-ray players as well as most other media players, these subtitles are always enabled by default. In VLC on the other hand, the concept of forced subtitles hasn't existed yet, so one had to always switch the subtitles on and back off again in order to understand what is said in a foreign language, which is pretty inconvenient.

These commits add initial support for forced subtitles for both DVD (spudec) and Blu-ray (pgssub via avcodec) by adding a switch in the corresponding decoder settings ("DVD subtitles" and "FFmpeg") to only display forced subtitles. It works with physical DVDs and Blu-rays as well as other containers (like mkv) using those subtitle formats.

I know the location of this implementation is far from optimal, but it is better than nothing and the best I could do with my limited knowledge of VLC's codebase.
In the long term, it would be better to add a switch in the subtitles toolbar to enable/disable only displaying forced subtitles.
The optimal implementation would be to emulate the behavior of physical players and enable only the forced subtitles for the given audio language automatically.

This closes the 15-year-old issue #1135.

Thank you very much for reading my detailed explanation!

TLDR: These changes make it possible to watch movies and series with foreign/alien languages like Game of Thrones without constantly turning subtitles on and off again to understand everything.