Bug 74822 [Preferences] Update spec for property change events
bundles/org.eclipse.jface/src/org/eclipse/jface/util/PropertyChangeEvent.java
*/
    private String propertyName;
    /**
     * The old value of the changed property, or <code>null</code> if
     * not known or not relevant.
     */
    private Object oldValue;
    /**
     * The new value of the changed property, or <code>null</code> if
     * not known or not relevant.
     */
    private Object newValue;
    /**
     * Creates a new property change event.
     *
     * @param source the object whose property has changed
     * @param property the property that has changed (must not be <code>null</code>)
     * @param oldValue the old value of the property, or <code>null</code> if none
     * @param newValue the new value of the property, or <code>null</code> if none
     */
    public PropertyChangeEvent(Object source, String property, Object oldValue,
            Object newValue) {
        super(source);
        Assert.isNotNull(property);
        this.propertyName = property;
        this.oldValue = oldValue;
        this.newValue = newValue;
    }
    /**
     * Returns the new value of the property.