chore: clean up mouse drag to resize/move logic a little bit

1. No longer fallback to use move(), since all known platforms
   support startSystemMove(), and it's unlikely going to work
   if it's not supported on a specific platform.
2. Fix titlebar move check missing `window()->`.

This amends a10bbb4e8d

app/mainwindow.h

@@ -119,7 +119,6 @@ private:
     ActionManager *m_am;
     PlaylistManager *m_pm;
 
-    QPoint m_oldMousePos;
     QPropertyAnimation *m_fadeOutAnimation;
     QPropertyAnimation *m_floatUpAnimation;
     QParallelAnimationGroup *m_exitAnimationGroup;