Make (Wicket) link and image picker work for Visual Editing

If the hippopicker plugin is enabled for a Rich Text field (which it is by default), make sure it successfully:

• uses the Wicket link picker to select a document/asset/image to link to
• uses the Wicket image picker to select the image (variant) to inline

Picking internal links and images should work in both the existing document editor as in the Visual Editor.