rm one Q_OBJECT macro; protect others against removal by explanatory notes (#521)

This resolves #521 (closed).

GUI/View/Numeric/DoubleSpinBox.h

@@ -19,7 +19,7 @@
 
 //! SpinBox for DoubleProperties, supporting units.
 class DoubleSpinBox : public QDoubleSpinBox {
-    Q_OBJECT
+    Q_OBJECT // required by qobject_cast (called via FormLayouter::widgetAt)
 public:
     //! Create a DoubleSpinBox with the information found in a DoubleProperty.
     //!