Bug 305703 [Viewers][CommonNavigator] remembered contributions can leak when using filters
bundles/org.eclipse.ui.navigator/src/org/eclipse/ui/internal/navigator/NavigatorContentService.java
* @return An instance of the content extension for the given extension id.
	 *         May return <b>null</b> if the id is invalid.
	 */
	public INavigatorContentExtension getContentExtensionById(
			String anExtensionId) {
		NavigatorContentDescriptor descriptor = CONTENT_DESCRIPTOR_REGISTRY
				.getContentDescriptor(anExtensionId);
		if (descriptor != null)
			return getExtension(descriptor);
		return null;
	}
	/**
	 * Search for extensions that declare the given element in their
	 * <b>triggerPoints</b> expression.
	 * 
	 * @param anElement
	 *            The element to use in the query
	 * @return The set of {@link INavigatorContentExtension}s that are
	 *         <i>visible</i> and <i>active</i> for this content service and
	 *         have a <b>triggerPoints</b> expression that is <i>enabled</i> for
	 *         the given element.
	 */
	public Set findContentExtensionsByTriggerPoint(Object anElement) {
		return findContentExtensionsByTriggerPoint(anElement, true, !CONSIDER_OVERRIDES);
	}
	/**
	 * Search for extensions that declare the given element in their
	 * <b>triggerPoints</b> expression.
	 * 
	 * @param anElement