ShowManager: improve playlist integration in timeslot table

Currently we can only click on the symbol and chose one of the existing playlists. We don't have any cue if and what is already set.

So we need:

1. a separate column showing the playlist title or number ✔ c9fa9f05
2. the ability to delete a playlist ✔ 607a243f
3. a quick link to the FileManager if we want to edit our playlists ✔ b7ef6f21
4. for future improvement:
   • an ad-hoc playlist generator / file-uploader