Add experimental support for minimization to side-bar

Auto-hide not working yet. Just minimization.
Internal for now. Will be public once finished.

src/private/widgets/TitleBarWidget_p.h

@@ -52,6 +52,7 @@ protected:
     void updateCloseButton() override;
     void updateMaximizeButton() override;
     void updateMinimizeButton() override;
+    void updateAutoHideButton() override;
 
      // The following are needed for the unit-tests
     bool isCloseButtonVisible() const override;
@@ -70,6 +71,7 @@ private:
     QAbstractButton *m_floatButton = nullptr;
     QAbstractButton *m_maximizeButton = nullptr;
     QAbstractButton *m_minimizeButton = nullptr;
+    QAbstractButton *m_autoHideButton = nullptr;
     QLabel *m_dockWidgetIcon = nullptr;
 };