Just FYI, folks: I've committed the change to the GoldenDict repository that makes the new UI mode (the search line is in the toolbar, at the top) the default one. So that new users will get the new UI by default.
If you wish to use the old mode for some reason, just use Menu -> View -> Search Pane, or just press Ctrl+S to re-enable your old sidebar. But I urge you to use the new UI mode and give it a try.