The Diff view in new UI will be using IFrames to display the diff.

Also, add changes in tests:

erp5_ui_test: Update history test according to use of diff gadget
erp5_web_renderjs_ui_test: Update tests according to new Diff view for Historical Revisions
testXHTML: Add Historical Comparison View to the list of listbox with no selection name
testXHTML: Simplify test_html_file and include way to ignore single html file