Playlist Folder Support

[maya-doshi]

Is your feature already implemented in the latest master?
No

Is your feature request related to a problem? Please describe.
Being able to organize playlists on spotify helps navigation a lot. It will be especially helpful when adding a song to a playlist

Describe the solution you'd like
Support playlist folders like on the official clients

Describe alternatives you've considered
Allowing custom ordering of playlists on spotify-player but that would always go out of sync with the official mobile client whenever a new playlist is added and generally be clumsy