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/api/MarkerField.java
*         <li>a negative number if the value of item1 is less than the
	 *         value of item2 for this field.
	 *         <li><code>0</code> if the value of item1 and the value of
	 *         item2 are equal for this field.
	 *         <li>a positive number if the value of item1 is greater than the
	 *         value of item2 for this field.
	 */
	public int compare(MarkerItem item1, MarkerItem item2) {
		return getValue(item1).compareTo(getValue(item2));
	}
	/**
	 * Generate the filter for the receiver from the configurationElement.
	 * 
	 * @return MarkerFieldFilter or <code>null</code>.
	 */
	public MarkerFieldFilter generateFilter() {
		try {
			if (configurationElement.getAttribute(ATTRIBUTE_FILTER_CLASS) == null)
				return null;
			Object filter = IDEWorkbenchPlugin.createExtension(
					configurationElement, ATTRIBUTE_FILTER_CLASS);
			if (filter == null)
				return null;
			MarkerFieldFilter fieldFilter = (MarkerFieldFilter) filter;
			fieldFilter.setField(this);
			return fieldFilter;
		} catch (CoreException e) {
			StatusManager.getManager().handle(e.getStatus());
			return null;
		}