Use this mixin for widgets that needs to know about and
keep track of their widget children. Suitable for widgets like BorderContainer
and TabContainer which contain (only) a set of child widgets.
It's not suitable for widgets like ContentPane
which contains mixed HTML (plain DOM nodes in addition to widgets),
and where contained widgets are not necessarily directly below
this.containerNode. In that case calls like addChild(node, position)
wouldn't make sense.