Partial update of DOMWidgetView styling and simplification of containers.

-The selection containers did not make use of the new `after_displayed` function. 
-The content of `WidgetContainer.render` is now split between `initialize` and `render`. The former calls DOMWidgetView.initialize, which registers its own partial updates. Besides that it is mainly code removal. 

I think that we are at the point where ContainerView is really as simple as it can get. It could be used as a base class for the popup and the different selection containers.