Bug 169025 Supports overridden on editor contribution
After contributting a new editor to the multiPageReportEditor, supports to enable the new editor to override a editor by assigning the ID.
UI/org.eclipse.birt.report.designer.ui.editors/src/org/eclipse/birt/report/designer/internal/ui/extension/EditorContributorManager.java
{
		IExtensionRegistry registry = Platform.getExtensionRegistry( );
		if ( registry == null )
		{
			return Collections.EMPTY_LIST;
		}
		IExtensionPoint extensionPoint = registry.getExtensionPoint( id );
		if ( extensionPoint == null )
		{
			return Collections.EMPTY_LIST;
		}
		return Arrays.asList( extensionPoint.getExtensions( ) );
	}
	/**
	 * @param newPoint
	 *            the extension point instance
	 * @param element
	 *            the configuration element
	 * @param className
	 *            the name of the class attribute
	 */
	private Object loadClass( IConfigurationElement element,
			String attributeName )
	{
		Object clazz = null;
		try
		{
			clazz = element.createExecutableExtension( attributeName );
		}
		catch ( CoreException e )