Implement LazyDisplay with insertionMethod

[carlosescri]

And remove insertionMethod from Display. If you re-render a Display appended, it's re-appended. The new pattern is LazyDisplay. It should use Display as the content widget by default but you could provide a method to get the widget instance as an option.

[carlosescri]

We have private code for this, we're going to make it public, just ask.