Bug 205049 Other plug-ins should be able to define default content generator
Currently only the content generator can define which plug-ins it can be the default for - we need a way for plug-ins to refer back to the marker support for thier own generator.
bundles/org.eclipse.ui.ide/src/org/eclipse/ui/internal/provisional/views/markers/MarkerContentGenerator.java
* @return MarkerComparator
	 */
	public MarkerComparator getComparator() {
		if (comparator == null) {
			MarkerField field = null;
			if (getCategoryGroup() != null)
				field = getCategoryGroup().getMarkerField();
			comparator = new MarkerComparator(field, getAllFields());
			comparator.restore(this.memento);
		}
		return comparator;
	}
	/**
	 * Get the id of the perspective this content generator is the default for.
	 * 
	 * @return String or <code>null</code>.
	 */
	public String getDefaultPerspectiveId() {
		return configurationElement
				.getAttribute(ATTRIBUTE_DEFAULT_FOR_PERSPECTIVE);
	}
	/**
	 * Return the currently enabled filters.
	 * 
	 * @return Collection of MarkerFieldFilterGroup
	 */
	public Collection getEnabledFilters() {
		if (enabledFilters == null) {
			enabledFilters = new HashSet();
			Iterator filtersIterator = getAllFilters().iterator();