Bug 249574 [Contributions] ExternalActionManager.addPropertyChangeListener() doesn't register the ListenerList
bundles/org.eclipse.jface/src/org/eclipse/jface/action/ExternalActionManager.java
/**
		 * Checks to see whether the given accelerator is being used by some
		 * other mechanism (outside of the menus controlled by JFace). This is
		 * used to keep JFace from trying to grab accelerators away from someone
		 * else.
		 * 
		 * @param accelerator
		 *            The accelerator to check -- in SWT's internal accelerator
		 *            format.
		 * @return <code>true</code> if the accelerator is already being used
		 *         and shouldn't be used again; <code>false</code> otherwise.
		 */
		public boolean isAcceleratorInUse(int accelerator);
		/**
		 * Checks whether the item matching this identifier is active. This is
		 * used to decide whether a contribution item with this identifier
		 * should be made visible. An inactive item is not visible.
		 * 
		 * @param identifier
		 *            The identifier of the item from which the active state
		 *            should be retrieved; must not be <code>null</code>.
		 * @return <code>true</code> if the item is active; <code>false</code>
		 *         otherwise.
		 */
		public boolean isActive(String identifier);
		/**
		 * Removes a listener from the object referenced by
		 * <code>identifier</code>. This identifier is specific to mechanism
		 * being used. In the case of the Eclipse workbench, this is the command
		 * identifier.