**This is for MPV playback backend only!** Since this only works in MPV
for now, the option is not shown if using ExoPlayer.
Adds a setting (the gear icon during playback) to adjust the subtitle
delay.
A negative delay shows subtitles sooner. A positive delay shows them
later. For example, if a subtitle is supposed to show between
1000ms-2000ms, a negative 250ms delay will show it at 750ms-1750ms
instead.
The delay is persisted between plays of the same media & subtitle track.
Ref: #12
