Handle comm_open messages containing new widget state

[philippjfr]

The way ipywidgets works when adding new models to an existing ipywidget is to send the widget state as part of the comm_open message that is sent on widget creation. To handle this correctly we now check whether a model already exists otherwise we pass the comm open message to the handle_comm_open handler on the WidgetManager.

[philippjfr]

@mattpap Could you merge this and cut a new release? The two changes do deserve a minor version bump.

[mattpap]

I was about to merge this when I saw the new commit. Is this fully ready?

[philippjfr]

Yes, sorry, ready now. The issue was that some libraries fail to correctly set defaults both server and client side so a server side default may still have to be sent to the client-side. In the case I was handling the LeafletControlModels were not correctly setting the _view_module clientside.