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
*         <i>visible</i> and <i>active</i> for this content service and
	 *         have a <b>possibleChildren</b> expression that is <i>enabled</i>
	 *         for the given element.
	 */
	public Set findDescriptorsWithPossibleChild(Object anElement,
			boolean toComputeOverrides) {
		
		Set result = new TreeSet(ExtensionPriorityComparator.INSTANCE);
		result.addAll(CONTENT_DESCRIPTOR_REGISTRY
				.findDescriptorsForPossibleChild(anElement, assistant,
						toComputeOverrides));
		return result;
	}
	/*
	 * (non-Javadoc)
	 * 
	 * @seeorg.eclipse.ui.internal.navigator.INavigatorContentService#
	 * onExtensionActivation(java.lang.String, java.lang.String, boolean)
	 */
	public void onExtensionActivation(String aViewerId,
			String[] aNavigatorExtensionId, boolean toEnable) {
		synchronized (this) {
			try {
				NavigatorContentDescriptor key;
				NavigatorContentExtension extension;
				for (Iterator iter = contentExtensions.keySet().iterator(); iter
						.hasNext();) {
					key = (NavigatorContentDescriptor) iter.next();
					INavigatorActivationService activation = getActivationService();
					if (!activation.isNavigatorExtensionActive(key.getId())) {