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
/**
	 * Process the content generator defined in the extension.
	 * 
	 * @param tracker
	 * @param extension
	 * @param element
	 */
	private void processContentGenerator(IExtensionTracker tracker,
			IExtension extension, IConfigurationElement element) {
		MarkerContentGenerator generator = new MarkerContentGenerator(element);
		generators.put(generator.getId(), generator);
		tracker.registerObject(extension, generator,
				IExtensionTracker.REF_STRONG);
	}
	/**
	 * Process the cross references after all of the extensions have been read.
	 * 
	 * @param groupingEntries
	 *            Mapping of group names to the markerGroupingEntries registered
	 *            for them
	 * @param attributeMappings
	 *            the markerAttributeGroupings found
	 */
	private void postProcessExtensions(Map groupingEntries,
			Collection attributeMappings) {
		processGroupingEntries(groupingEntries);
		processAttributeMappings(attributeMappings);
		postProcessContentGenerators();
	}
	/**