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
IConfigurationElement[] types = element
				.getChildren(MARKER_TYPE_REFERENCE);
		String[] ids = new String[types.length];
		for (int i = 0; i < ids.length; i++) {
			ids[i] = types[i].getAttribute(MarkerSupportConstants.ATTRIBUTE_ID);
		}
		return ids;
	}
	/*
	 * (non-Javadoc)
	 * 
	 * @see org.eclipse.core.runtime.dynamichelpers.IExtensionChangeHandler#addExtension(org.eclipse.core.runtime.dynamichelpers.IExtensionTracker,
	 *      org.eclipse.core.runtime.IExtension)
	 */
	public void addExtension(IExtensionTracker tracker, IExtension extension) {
		Map groupingEntries = new HashMap();
		Set attributeMappings = new HashSet();
		processExtension(tracker, extension, groupingEntries, attributeMappings);
		postProcessExtensions(groupingEntries, attributeMappings);
	}
	/**
	 * Get the collection of currently registered filters.
	 * 
	 * @return Collection of ProblemFilter
	 */
	public Collection getRegisteredFilters() {
		Collection filteredFilters = new ArrayList();
		Iterator registeredIterator = registeredFilters.values().iterator();
		while (registeredIterator.hasNext()) {
			ProblemFilter next = (ProblemFilter) registeredIterator.next();