Allow users to return back to XWiki

The goal is to allow users to navigate easily between Cristal and XWiki.

Tasks:

• Propose a design for the integration of the new navigation action in the UI.
• Check the Cristal has the sufficient user interface extensions to support the proposal (and implement the missing extension if needed). 
• Define a Cristal Integration specific extension to inject the new UI element and add it to the loaded extensions in the Cristal integration bundle