RBAC: role combination doesn't work

[This issue was in fact found on 2013-07-02 and is being filled now only for tracking purposes.]

Combining multiple roles doesn't work, as ManagementPermissionCollection(CombinationManagementPermission) will never imply a ManagementPermissionCollection(SimpleManagementPermission). See TODOs in DefaultPermissionFactory and ManagementPermissionCollection.