widgets: Use list box in Add to Playlist Dialog
view: Don't add new playlist if Playlists view is not yet initialized

This will prevent new playlists to be shown multiple times if they are
created before the Playlists view is shown.
widgets: Add the playlist song to the correct model

This ensures that if we move to a another playlist while the previous
playlist is still loading, the songs won't get mixed up with the songs
of the new playlist.