feat: add option to provide a titlebar (default on)

app/settingsdialog.h

@@ -24,6 +24,7 @@ public slots:
 private:
     QCheckBox * m_stayOnTop = nullptr;
     QCheckBox * m_useBuiltInCloseAnimation = nullptr;
+    QCheckBox * m_showTitleBar = nullptr;
     QCheckBox * m_useLightCheckerboard = nullptr;
     QCheckBox * m_loopGallery = nullptr;
     QCheckBox * m_autoLongImageMode = nullptr;