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/PreviewWizardPage.java
private void showNullPreviewer() {
		fCurrentDescriptor= null;
		fCurrentPreviewViewer= fNullPreviewer;
		fPreviewContainer.showPage(fCurrentPreviewViewer.getControl());
	}
	/**
	 * Returns <code>true</code> if the preview page will show any changes when
	 * it becomes visibile. Otherwise <code>false</code> is returned.
	 * 
	 * @return whether the preview has changes or not
	 */
	public boolean hasChanges() {
		if (fChange == null)
			return false;
		if (fChange instanceof CompositeChange)
			return ((CompositeChange)fChange).getChildren().length > 0;
		return true;
	}
	
	
	
	private Collection collectGroupCategories() {
		Set result= new HashSet();
		collectGroupCategories(result, fChange);
		return result;
	}
	
	private void collectGroupCategories(Set result, Change change) {
		if (change instanceof TextEditBasedChange) {
			TextEditBasedChangeGroup[] groups= ((TextEditBasedChange)change).getChangeGroups();