Add subtitle mvp

[Offlinemaker]

Adds extremely basic subtitle functionality.

This basically uses the default subtitle stream of the original video file if toggled on. FFMPEG does seemingly not support seeking with subtitles so the related options need to be added to the output file instead. This means that everything that comes before the clip must be rendered as well. Therefore the disclaimer of "Will take way longer" next to the checkbox.

As basic as it may be, this implementation should cover ~80% of my library and I think it may be the same for many other people.