cross repository type browsing. The Node Browser seems to work
happily with this. The UI can traverse a mounpoint but errors out
when trying to browse further down into an AVM store, because its
being naughty and peeking into NodeRefs.
git-svn-id: https://svn.alfresco.com/repos/alfresco-enterprise/alfresco/BRANCHES/WCM-DEV2/root@3649 c4b6b30b-aa2e-2d43-bbcb-ca4b014f7261