[WIP] New playlist functionality (Final)

: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)
- [x] Play the next song in the playlist after the current song
- [x] Add songs by drag&drop
- [x] Remove songs from playlists

Working on:

- [ ] Design improvements

To do:

- [ ] Unit tests
- [ ] Documentation

Playlists will:

- [x] Repeat playlist songs

Playlists may:

- [ ] be sorted by dragging

... and will be updated