Bug 299438 [CommonNavigator] CNF viewer state non properly reset when NCEs are activated or deactivated
The map of the objects to the NCEs should be reset whenever the NCEs associated with the viewer to change to avoid having stale label information. This has shown up when looking at the CNF tests that change the viewer.
bundles/org.eclipse.ui.navigator/src/org/eclipse/ui/internal/navigator/NavigatorContentService.java
* 
	 * @param element
	 *            The element to forget.
	 */
	public void forgetContribution(Object element) {
		contributionMemory.remove(element);
	}
	
	/**
	 * @param element
	 * @return the remembered NavigatorContentDescriptor
	 */
	public NavigatorContentDescriptor getContribution(Object element)
	{
		NavigatorContentDescriptor desc = (NavigatorContentDescriptor) contributionMemory.get(element);
		contributionMemory.remove(element);
		return desc;
	}
	
	/**
	 * 
	 * @param element
	 *            The element contributed by the descriptor to be returned
	 * @return The descriptor that contributed the element or null.
	 * @see #findContentExtensionsByTriggerPoint(Object)
	 */
	public synchronized NavigatorContentDescriptor getSourceOfContribution(Object element) {
		if (element == null)
			return null;
		if (structuredViewerManager == null)