SLING-2985 : basic add/rename/delete operations on child-nodes inside a .content.xml in the content-browser

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1520037 13f79535-47bb-0310-9956-ffa450edef68
SLING-3009 : avoid a npe

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1520038 13f79535-47bb-0310-9956-ffa450edef68