revert: remove auto long image mode

- remove auto long image mode.
- there is no "auto" requirement for this.
- user can utilize fit by height/width to achieve the same goal by manually click them.
- some optimization code about graphic view are preserved.

app/settingsdialog.h

@@ -26,7 +26,6 @@ private:
     QCheckBox * m_useBuiltInCloseAnimation = nullptr;
     QCheckBox * m_useLightCheckerboard = nullptr;
     QCheckBox * m_loopGallery = nullptr;
-    QCheckBox * m_autoLongImageMode = nullptr;
     QCheckBox * m_svgTiny12Only = nullptr;
     QComboBox * m_doubleClickBehavior = nullptr;
     QComboBox * m_mouseWheelBehavior = nullptr;