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
* 
	 * @param perspective
	 * @return {@link MarkerContentGenerator} or <code>null</code>.
	 */
	public MarkerContentGenerator generatorFor(
			IPerspectiveDescriptor perspective) {
		String id = perspective.getId();
		Iterator generatorIterator = generators.values().iterator();
		while (generatorIterator.hasNext()) {
			MarkerContentGenerator generator = (MarkerContentGenerator) generatorIterator
					.next();
			if (id.equals(generator.getDefaultPerspectiveId()))
				return generator;
		}
		return getDefaultGenerator();
	}
	/**
	 * Return the default content generator.
	 * 
	 * @return MarkerContentGenerator
	 */
	private MarkerContentGenerator getDefaultGenerator() {
		return (MarkerContentGenerator) generators
				.get(DEFAULT_PROBLEMS_GENERATOR_ID);
	}
	/**
	 * Get the markerGroup associated with categoryName
	 * 
	 * @param categoryName
	 * @return FieldMarkerGroup or <code>null</code>