Add e_collection_backend_claim_all_resources().

Add e_collection_backend_claim_all_resources().

Claims all previously used sources that have not yet been claimed by
e_collection_backend_new_child() and returns them in a GList.  Note
that previously used sources can only be claimed once, so subsequent
calls to this function for backend will return NULL.

The backend is then expected to compare the returned list with a
current list of resources from a remote server, create new ESource
instances as needed with e_collection_backend_new_child(), discard
unneeded ESource instances with e_source_remove(), and export the
remaining instances with e_source_registry_server_add_source().