[WIP] New playlist functionality

:construction: Work in progress!

Ready:

- [x] Sidebar with playlists
- [x] Hardcoded playlists
- [x] Hardcoded song display
- [x] Playlist - Database connection
- [x] Create basic playlist functionalities (create, edit, delete playlist)

Working on:

- [ ] Play the next song in the playlist after the current song

To do:

- [ ] Unit tests
- [ ] Add songs by drag&drop

Playlists will:

- [ ] Repeat playlist songs

Playlists may:

- [ ] be sorted by dragging

... and will be updated
Yes. It is more flexible. What if you have more than 500 tracks in your list. In you old way the text field has a total length of 256, if you separate the track_ids by colon you have to substract the half of the possible characters. 128 IDs if they are single-characters. But we use numbers -> so they have to be 2 or more digits long -> around 100 tracks per playlist.