webui: widget initialization
    
    - used `ctor_init` instead of `init` to avoid name collision with
      existing logic
    - `ctor_init` is called right after widget instantiation. Basically support
      better inheritance for the old class system which doesn't have proper
      contructors
    
    https://fedorahosted.org/freeipa/ticket/4402
    
    Reviewed-By: Endi Sukma Dewata <edewata@redhat.com>