This feature needed to change quite some code in the way children are
fetched in CMIS UCP to avoid running several times the same HTTP
requests.

The new URLs will find the following scheme:
 + Children of vnd.libreoffice.cmis+atom://admin@http-encoded-binding-url-no-repoid/
   are the repositories listed by the binding URL
 + Children of vnd.libreoffice.cmis+atom://admin@http-encoded-binding-url-no-repoid/repoid
   are the root folder of the repository

Change-Id: I405d88c82e6fc9f1110a84165a151257c4ce60db