[request] autosave the playlist

[korakinos]

Overview Description: I would like the option (or indeed have it as the default behaviour) for the playlist to be auto-saved on quit and automatically reloaded on program start.

Steps to Reproduce:

1. Add a file to the playlist in celluloid.
2. Close celluloid.
3. Start celluloid.

Actual Results: Celluloid starts with an empty playlist.

Expected Results: The old playlist is restored.

Version: 0.20 (flatpak)

Additional Information: Together with mpv's option --save-position-on-quit this would make celluloid uniquely suitable for listening to tracks so long that I might stop and reboot my computer before one track has ended. With this behaviour, I would just need to restart celluloid and would be able to resume playing without having to re-open the respective file(s).