refactor: opacity helper

bottombuttongroup.h

@@ -4,12 +4,14 @@
 #include <QAbstractButton>
 #include <QGroupBox>
 
+class OpacityHelper;
 class BottomButtonGroup : public QGroupBox
 {
     Q_OBJECT
 public:
     explicit BottomButtonGroup(QWidget *parent = nullptr);
 
+    void setOpacity(qreal opacity, bool animated = true);
     void addButton(QAbstractButton *button);
 
 signals:
@@ -19,6 +21,9 @@ signals:
     void zoomOutBtnClicked();
     void toggleCheckerboardBtnClicked();
     void rotateRightBtnClicked();
+
+private:
+    OpacityHelper * m_opacityHelper;
 };
 
 #endif // BOTTOMBUTTONGROUP_H