DetailsWidget: partially revert removing of obsolete code

Used on the mac.

Change-Id: I6d96d9c706b1f4b94482f31cd87c4db48792b062
Reviewed-by: Daniel Teske <daniel.teske@nokia.com>

src/libs/utils/detailsbutton.h

@@ -46,6 +46,7 @@ class QTCREATOR_UTILS_EXPORT FadingPanel : public QWidget
 {
 public:
     FadingPanel(QWidget *parent = 0);
+    void setOpacity(qreal value);
     void fadeTo(qreal value);
 protected:
     QGraphicsOpacityEffect *m_opacityEffect;