Simpler interface for Jukebox users

* put jukebox mode on the main menu
* switch between UltraSonic Android and UltraSonic Android-jukebox client string

The client string is for a version of Subsonic MusicCabinet I'm working on that allows playing using Spotify in Jukebox mode, but no streaming. I want to know when I'm not browsing in jukebox mode so I don't return Spotify albums. It's similar to what Subsonic does on the server side currently, so I figured it might be useful to others who want to add options for configuring different streaming and jukebox behavior.