do not remove body children when navigating

[jstrimpel]

When we navigate between routes that do not share the same layout or do not have a layout we are clear out the body. We should only remove the root controller's element and append the new root controller's element.

The reason being is that an application level widget may have appended markup to the body. This is commonly done for widgets that need to position themselves absolutely and have the highest z-index, e.g., a modal dialog.