Add "Cut subtitle after first line"

This feature lets you cut the selected subtitle after the first line at
the current track position. The first line becomes the left subtitle
before the track position, and the rest becomes the right subtitle after
the track position. It then selects the right hand subtitle.

Before:
```
    | Subtitle 1\nSubtitle 2                     |
                                   ^
			     Track position
```

After:
```
    | Subtitle 1                   || Subtitle 2 |
```

This makes it much easier to split up subtitles where you have a
transcription with one subtitle per line. This comes from a couple of
common scenarios:
  1) You have put the audio through an external  transcription service
  2) You are adding subtitles to a song that you have the lyrics for