core (xmlui): container now keep a list of children

src/tools/xml_tools.py

@@ -412,6 +412,7 @@ class Element(object):
         @parent: parent element
         """
         assert self.type is not None
+        self.children = []
         if not hasattr(self, 'elem'):
             self.elem = parent.xmlui.doc.createElement(self.type)
         self.xmlui = xmlui
@@ -427,6 +428,7 @@ class Element(object):
         """
         self.elem.appendChild(child.elem)
         child.parent = self
+        self.children.append(child)
         return child