sort playlist: add 'album artist' to the sort options

NEWS

@@ -1,3 +1,6 @@
+ncmpcpp-0.7_beta2 (????-??-??)
+* Playlist sorting dialog now contains 'Album artist' option.
+
 ncmpcpp-0.7_beta1 (2015-11-04)
 
 * Visualizer has now support for multiple colors (visualizer_color configuration variable takes the list of colors to be used).

src/sort_playlist.cpp

@@ -50,6 +50,9 @@ SortPlaylistDialog::SortPlaylistDialog()
 	w.addItem(Entry(std::make_pair("Artist", &MPD::Song::getArtist),
 		std::bind(&Self::moveSortOrderHint, this)
 	));
+	w.addItem(Entry(std::make_pair("Album artist", &MPD::Song::getAlbumArtist),
+		std::bind(&Self::moveSortOrderHint, this)
+	));
 	w.addItem(Entry(std::make_pair("Album", &MPD::Song::getAlbum),
 		std::bind(&Self::moveSortOrderHint, this)
 	));