Add video playback options for 500 ms before/after subtitle lines

[forksama]

Background

I often need to precisely time subtitle lines and therefore need a convenient way to view the video and audio around each line. However, after checking the documentation, I found that this functionality is not currently available (only audio has a similar feature). Further research in the forums showed that other users have expressed the same need.

Modification

Added four new playback options:

• Play 500 ms from the start of the current line: video/play/line/begin
• Play 500 ms from the end of the current line: video/play/line/end
• Play 500 ms before the start of the current line: video/play/line/before
• Play 500 ms after the end of the current line: video/play/line/after