Make SphericalDetectorItem::P_ALPHA_AXIS private

GUI/Models/SphericalDetectorItem.h

@@ -22,9 +22,9 @@ class BasicAxisItem;
 class BA_CORE_API_ SphericalDetectorItem : public DetectorItem {
 private:
     static const QString P_PHI_AXIS;
+    static const QString P_ALPHA_AXIS;
 
 public:
-    static const QString P_ALPHA_AXIS;
     SphericalDetectorItem();
 
     int xSize() const override;