widgets: Load list in PlaylistDialog from Tracker
query: Add query for deleting playlists
playlists: Rewrite delete_playlist to use Tracker queries
view: Pass media object to delete_playlist
query: Use entry id for Playlist entries
query: Add queries for adding songs to a playlist
grilo: Add method for getting playlist song using id
playlists: Rewrite add_to_playlist to use Tracker queries
view: Store current playlist as media object

Also updates callback for song-added-to-playlist signal to match recent
changes to Playlists.add_to_playlist method.
view: Store selected items as media objects
query: Add query for removing songs from a playlist
playlists: Rewrite remove_from_playlist to use Tracker queries
view: Update callback for song-removed-from-playlist signal