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/titlebar.h

@@ -54,7 +54,6 @@ private:
     bool m_closeHovered = false;
     bool m_closePressed = false;
     bool m_dragPending = false;
-    QPoint m_moveStartPos;
 };
 
 #endif // TITLEBAR_H