webui: ContainerMixin
    
    A mixin which implements widget storing logic. Similar logic is already implemented
    in details facet and dialog.
    
    Long term goal is to replace that with this one.
    
    Separating the logic into mixin makes it usable in other components.
    
    https://fedorahosted.org/freeipa/ticket/3903
    
    Reviewed-By: Adam Misnyovszki <amisnyov@redhat.com>