maintenance: adding all models as types for direct access

If each model is registered as a QML type, we can pass the model back
to QML from each proxy model to have more direct access to it. This
enables us to use it's functions and properties better.

src/cpp/songsqlmodel.cpp

@@ -757,6 +757,10 @@ QVariantMap SongProxyModel::getSong(const int &row) {
   return song;
 }
 
+void SongProxyModel::newSong() {
+  m_songModel->newSong();
+}
+
 void SongProxyModel::deleteSong(const int &row) {
   auto model = qobject_cast<SongModel *>(sourceModel());
   model->removeItem(row);

src/cpp/songsqlmodel.h

@@ -131,6 +131,7 @@ public:
 
 public slots:
   Q_INVOKABLE QVariantMap getSong(const int &row);
+  Q_INVOKABLE void newSong();
   Q_INVOKABLE void deleteSong(const int &row);
   Q_INVOKABLE void deleteSongs(const QVector<int> &rows);
   Q_INVOKABLE QStringList getLyricList(const int &row);