Add the Qt window module as an alternative to the SDL2 module

Att the Qt window module as an alternative to the SDL2 module

Description is TBD

src/module/qt_window/mpvwidget.cc

@@ -252,7 +252,7 @@ MpvWidget::update_window_title()
 {
     struct kara_metadata kara_mdt;
     int changed_kara = 0;
-    char *kara_title = NULL;
+    char *kara_title = nullptr;
     char window_title[LKT_LINE_MAX];
 
     if (database_queue_current_kara(m_db, &kara_mdt, &changed_kara)) {