Bug 93400 LTK UI should provide official API for structural content in the preview page
org.eclipse.ltk.ui.refactoring/src/org/eclipse/ltk/internal/ui/refactoring/TextEditChangeElement.java
public TextEditBasedChangeGroup getChangeGroup() {
		return fChangeGroup;
	}
	
	public Object getModifiedElement() {
		return fChangeGroup;
	}
	
	public Change getChange() {
		return null;
	}
	
	public ChangePreviewViewerDescriptor getChangePreviewViewerDescriptor() throws CoreException {
		DefaultChangeElement element= getDefaultChangeElement();
		if (element == null)
			return null;
		return element.getChangePreviewViewerDescriptor();
	}
	
	public void feedInput(IChangePreviewViewer viewer, List categories) throws CoreException {
		DefaultChangeElement element= getDefaultChangeElement();
		if (element != null) {
			Change change= element.getChange();
			if (change instanceof TextEditBasedChange) {
				IRegion range= getTextRange(this);
				ChangePreviewViewerInput input= null;
				if (range != null) {
					input= TextEditChangePreviewViewer.createInput(change, new TextEditBasedChangeGroup[] {fChangeGroup}, range);
				} else {
					input= TextEditChangePreviewViewer.createInput(change, fChangeGroup, 2);