Store the current document when entering the Admin Center, and restore it when exiting.

For the BIRT addon, we have the listing of Report Models available in the Admin Center, and we wanted to display the details of a Report without exiting the Admin Center: we use the default tabs and document layouts (so we need a currentDocument to be set to the Report model.

=> we need to store the last visited document, to be able to restore it when exiting the Admin Center (instead of still being on the Report Model we selected).