Add const/references to all signal parameters

src/dialogs/edittagdialog.h

@@ -77,7 +77,7 @@ class EditTagDialog : public QDialog {
   void accept() override;
 
  signals:
-  void Error(QString message);
+  void Error(const QString &message);
 
  protected:
   bool eventFilter(QObject *o, QEvent *e) override;

src/dialogs/trackselectiondialog.h

@@ -54,8 +54,8 @@ class TrackSelectionDialog : public QDialog {
   void accept() override;
 
  signals:
-  void Error(QString);
-  void SongChosen(Song original_song, Song new_metadata);
+  void Error(const QString &error);
+  void SongChosen(const Song &original_song, const Song &new_metadata);
 
  private slots:
   void UpdateStack();