Deal with duplicate TMarks

[fijam]

md-rec will insert a TMark at the end of a track and the recorder may insert another if it detects long enough silence resulting in duplicates. Two ways to deal with that:

1. Just delete the extra TMarks manually later.
2. Add a 2s silence track between all items on the playlist and let the player handle TMarks.

#2 needs a mode where no TMarks are entered programatically and empty track names are not entered.

[fijam]

Simple implementation in 6687961 use with ./md-rec.py --no-tmarks

The 2-second silent track must must be named Silence by artist Silence. Currently hard-coded, You can use https://github.com/fijam/md-rec/blob/main/silence.mp3