Adjust SRT Subtitle Offset in the UI

[FordGuy61]

@tescophil

the real time offset adjustment is not available when I use an .srt file alongside the AV file on my server.

Is the video transcoding?

Subtitle offset is available only when the video is direct playing/streaming.

If the video is transcoding, subtitle offset is not available.

See: Subtitle Offset has disappeared on nVidia Shield - #5 by morganj