Song: add pretty-printers for EBU R 128 Integrated Loudness and Loudness Range fields

They end up being used in a quite a number of places later on,
it makes sense to have them in a common place.

Integrated Loudness (LUFS) is *usually* negative, so we really want to
always print a sign. But Loudness Range is non-negative.

I think it makes sense to print one or at most two decimal places for these.

src/core/song.h

@@ -225,6 +225,12 @@ class Song {
 
   const QUrl &stream_url() const;
 
+  static QString Ebur128LoudnessLUFSToText(const std::optional<double> &v);
+  QString Ebur128LoudnessLUFSToText() const;
+
+  static QString Ebur128LoudnessRangeLUToText(const std::optional<double> &v);
+  QString Ebur128LoudnessRangeLUToText() const;
+
   // Setters
   void set_id(const int id);
   void set_valid(const bool v);