refactor AdvancedSettingsDialog into own file and class

SongDialog.h

@@ -0,0 +1,30 @@
+#ifndef SONGDIALOG_H
+#define SONGDIALOG_H
+
+#include <QDialog>
+#include <QString>
+
+namespace Ui {
+class SongDialog;
+}
+
+class SongDialog : public QDialog
+{
+    Q_OBJECT
+
+public:
+    explicit SongDialog(QWidget *parent = nullptr, const QString &caption = "", const QString &lyrics = "");
+    ~SongDialog();
+    
+    QString getCaption() const;
+    QString getLyrics() const;
+    
+private slots:
+    void on_okButton_clicked();
+    void on_cancelButton_clicked();
+
+private:
+    Ui::SongDialog *ui;
+};
+
+#endif // SONGDIALOG_H