How to use the pygubu.builder function in pygubu

def populate_tree(self, master, parent, element,from_file=False):
        """Reads xml nodes and populates tree item"""

        data = WidgetDescr(None, None)
        data.from_xml_node(element)
        cname = data.get_class()
        uniqueid = self.get_unique_id(cname, data.get_id())
        data.set_property('id', uniqueid)

        if cname in builder.CLASS_MAP:
            pwidget = self._insert_item(master, data,from_file=from_file)
            xpath = "./child"
            children = element.findall(xpath)
            for child in children:
                child_object = child.find('./object')
                cwidget = self.populate_tree(pwidget, child, child_object,from_file=from_file)

            return pwidget
        else:
            raise Exception('Class "{0}" not mapped'.format(cname))

self.bindings_tree = self.builder.get_object('bindingstree')

        #Preview
        previewc = self.builder.get_object('preview_canvas')
        self.previewer = PreviewHelper(previewc)
        #tree editor
        self.tree_editor = WidgetsTreeEditor(self)

        self.builder.connect_callbacks(self)

        #Status bar
        self.statusbar = self.builder.get_object('statusbar')
        handler = StatusBarHandler(self.statusbar)
        handler.setLevel(logging.INFO)
        logger.addHandler(handler)
        pygubu.builder.logger.addHandler(handler)

        #app grid
        self.set_resizable()

        #
        #Application bindings
        #
        master = self.master
        master.bind_all(
            '',
            lambda e: self.on_file_menuitem_clicked('file_new'))
        master.bind_all(
            '',
            lambda e: self.on_file_menuitem_clicked('file_open'))
        master.bind_all(
            '',

def create_treelist(self):
        root_tagset = set(('tk', 'ttk'))

        #create unique tag set
        tagset = set()
        for c in builder.CLASS_MAP.keys():
            wc = builder.CLASS_MAP[c]
            tagset.update(wc.tags)
        tagset.difference_update(root_tagset)

        treelist = []
        for c in builder.CLASS_MAP.keys():
            wc = builder.CLASS_MAP[c]
            ctags = set(wc.tags)
            roots = (root_tagset & ctags)
            sections = (tagset & ctags)
            for r in roots:
                for s in sections:
                    key = '{0}>{1}'.format(r, s)
                    treelist.append((key, wc))

        #sort tags by label
        def by_label(t):