Playlist: add newly-added columns

Still mostly boilter-plate-y. It is somewhat interesting to see that info
in playlist view, so add the two fileds as columns.

At least for Integrated loudness, since it's normally negative,
we need to add a specialized Delegate.

src/playlist/playlist.h

@@ -131,6 +131,8 @@ class Playlist : public QAbstractListModel {
     Column_Mood,
     Column_Rating,
     Column_HasCUE,
+    Column_EBUR128IntegratedLoudness,
+    Column_EBUR128LoudnessRange,
     ColumnCount
   };