Builders and collections for fields and widgets
    
    Introduced IPA.field_container and IPA.widget_container.
    
    IPA.field_container: collection for fields. Can set logical container (facet, dialog...) to fields.
    IPA.widget_container: collection for widgets. Has basic searching capability withing widget tree.
    
    Introduced field_builder, widget_builder, section_builder, details_builder. All are used for building fields and widgets. Field_builder and widget_builder have the main building logic. Section_builder can create content based on current section spec. Details builder defines a strategy for building content.
    
    https://fedorahosted.org/freeipa/ticket/2040