Revert "DetailsWidget: Remove unused obsolete code"

This reverts commit 496ed22159.
Turns out that the code is used by a QPropertyAnimation.

Change-Id: Ia389710ff9e6f90f898f79cecf075fdf0a4ed6a5
Reviewed-by: Daniel Teske <daniel.teske@nokia.com>

src/libs/utils/detailsbutton.h

@@ -55,11 +55,14 @@ protected:
 class QTCREATOR_UTILS_EXPORT DetailsButton : public QAbstractButton
 {
     Q_OBJECT
+    Q_PROPERTY(float fader READ fader WRITE setFader)
 
 public:
     DetailsButton(QWidget *parent = 0);
 
     QSize sizeHint() const;
+    float fader() { return m_fader; }
+    void setFader(float value) { m_fader = value; update(); }
 
 protected:
     void paintEvent(QPaintEvent *e);