Refactor cover api & Playlist model

Some refactoring for the cover API :
- All the methods now use an AlbumInfo instead of strings
- The objects from MPD (Album, Aritst) provide this AlbumInfo object
- The Playlist data model was made of strings making it hard to plug into this object model. I refactored it to use an object model (ListAdapter with Music ...). I hope this will make future modifications safer.
- The stream URL is now always displayed in the playlist (on the second line) to know the radio you  are listening to even if the tag changed during the play.
- Use the Android Textutils to get rid of custom methods for nothing (I didn't know this class)
- Add progress bars when you change the cover (album view ...)
- Update the nowplaying view when you change cover from the library view


I did not push this because the refactoring on the playlist is kind of big.I wanted to know if abarisain is OK with it. I tested it and it seems to work nicely. I think it will made future modifications easier  (easy to make a mistake on a string name ...)