Uniquify spotify playlist names

[kingosticks]

Implements #114 (comment)

• Appends the playlist owner to those playlists owned by other users i.e. followed playlists
  • Left playlists owned by the current Spotify user alone - presumably they shouldn't appear any different to LocalBackend playlists
• Appends an incrementing number to playlists with the same name
  • First occurrence of a name is unchanged
  • Looking again I've realised this isn't the place to do this when dealing with multiple backends, perhaps I should remove this bit?

[jodal]

I also think that identically named playlists isn't the Spotify backend's problem as it identifies playlists uniquely by URI. It's an issue the MPD frontend should solve on its own.

You don't have to do anything to remove the identical name part. I'll just merge the first commit tonight and let the other one be.

[kingosticks]

OK great, thanks!

[jodal]

Merged in 1c0601e.