Support multiple Spotify accounts

[janwillemb]

As a familymember
I want to select my own Spotify account from a list of family accounts
So I can stream my own playlists and tracks.

Steps

• user opens mopidy client > Library
• system presents "Spotify" in the list
• user selects "Spotify"
• if there is only 1 Spotify account in the config:
  • system presents "Top Lists, Your Music, Play Lists" as usual
• if there are multiple accounts in the config:
  • system presents the usernames (or friendly names)
  • user selects a username
  • system presents "Top Lists, Your Music, Play Lists" for that user
• the rest works as usual

Config

• config should be backwards compatible
• add extra accounts by repeating all config vars and postfixing them with _<number>, so username_2 =..., password_2 = ..., etc.
• allow gaps in the numbering for ease of use