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
for (int i= 0; i < groups.length; i++) {
				result.addAll(groups[i].getGroupCategorySet().asList());
			}
		} else if (change instanceof CompositeChange) {
			Change[] children= ((CompositeChange)change).getChildren();
			for (int i= 0; i < children.length; i++) {
				collectGroupCategories(result, children[i]);
			}
		}
	}
	
	private void setActiveGroupCategory(GroupCategory category) {
		if (fActiveGroupCategories == null) {
			fActiveGroupCategories= new ArrayList(1);
		} else {
			fActiveGroupCategories.clear();
		}
		fActiveGroupCategories.add(category);
		fTreeViewer.setGroupCategory(fActiveGroupCategories);
		fTreeViewerPane.setText(Messages.format(
			RefactoringUIMessages.PreviewWizardPage_changes_filtered, 
			category.getName()));
	}
	
	private void clearGroupCategories() {
		fActiveGroupCategories= null;
		fTreeViewer.setGroupCategory(null);
		fTreeViewerPane.setText(RefactoringUIMessages.PreviewWizardPage_changes); 
	}
}