Now indexToSense and senseToIndex work with XYZ configs.

- Make static MusicPlayerApp.getConfig() globally available.
- Fill in artist and album from path name of media file.
- Queue is now array of SongInfo, like Library.
- when db is restored successfully, the restore file is renamed so it won't be auto-restored next time.
- Back button now exits app. However, it now always stops playing and exits.
- crash on resume, restore, etc. fixed.