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.cpp

@@ -685,6 +685,26 @@ QString Song::SampleRateBitDepthToText() const {
 
 }
 
+QString Song::Ebur128LoudnessLUFSToText(const std::optional<double> &v) {
+
+  if (!v) return QObject::tr("Unknown");
+
+  return QString::asprintf("%+.2f ", *v) + QObject::tr("LUFS");
+}
+QString Song::Ebur128LoudnessLUFSToText() const {
+  return Ebur128LoudnessLUFSToText(d->ebur128_integrated_loudness_lufs_);
+}
+
+QString Song::Ebur128LoudnessRangeLUToText(const std::optional<double> &v) {
+
+  if (!v) return QObject::tr("Unknown");
+
+  return QString::asprintf("%.2f ", *v) + QObject::tr("LU");
+}
+QString Song::Ebur128LoudnessRangeLUToText() const {
+  return Ebur128LoudnessRangeLUToText(d->ebur128_loudness_range_lu_);
+}
+
 QString Song::PrettyRating() const {
 
   float rating = d->rating_;

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);