webui: facet container
    
    A widget which servers as container for facets. FacetContainer is a base
    class. App is specialization.
    
    Doing this abstraction will allow us to implement various facet containers.
    
    https://fedorahosted.org/freeipa/ticket/3903
    
    Reviewed-By: Adam Misnyovszki <amisnyov@redhat.com>