Improve source viewer to handle object-based documents

Currently, viewer=code displays the document source only when the page contains content in the document body. However, some XWiki documents store their meaningful data in XObjects (Panels, Menus, etc) rather than in the page content itself. In such cases, viewer=code returns a blank view, making it hard to inspect the actual stored data, users left with option to check the content in object edit mode. It would be useful to extend viewer=code to also expose XObject-based content for such documents.