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/views/markers/internal/MarkerSupportRegistry.java
/**
	 * The label attribute
	 */
	static final String LABEL = "label"; 
	private static final String MARKER_ATTRIBUTE_GROUPING = "markerAttributeGrouping";
	private static final String DEFAULT_GROUPING_ENTRY = "defaultGroupingEntry";
	private static final String MARKER_TYPE = "markerType";
	private static final String PRIORITY = "priority"; 
	private static final String MARKER_GROUPING_ENTRY = "markerGroupingEntry"; 
	private static final Object SEVERITY_ID = "org.eclipse.ui.ide.severity";
	/**
	 * The tag for content generators.
	 */
	static final String MARKER_CONTENT_GENERATOR = "markerContentGenerator"; 
	private static final Object MARKER_FIELD = "markerField"; 
	private static final Object DEFAULT_PROBLEMS_GENERATOR_ID = "org.eclipse.ui.ide.problemsGenerator"; 
	private static final String ATTRIBUTE_CLASS = "class"; 
	private static MarkerSupportRegistry singleton;
	
	private static Object creationLock = new Object();
	/**
	 * Get the instance of the registry.
	 * 
	 * @return MarkerSupportRegistry
	 */
	public static MarkerSupportRegistry getInstance() {
		if (singleton == null) {
			synchronized (creationLock) {
				if (singleton == null) {