Enable widget instanciation from front-end.
Make Python push initial state.
Also added initial state push callback.
Fixed typo.
Rebase fixes.
Register the Widget target with the comm manager after construction.
Address @minrk 's review comments.
Clarified API for the create_model function,
Also added a test!
Don't eat comm close error silently.
Address some more review comments...
s/target_name/widget_class
JS test fix
Rebase fixes
Merge pull request #6664 from jdfreder/symmetric_widg

Allow widgets to be constructed from Javascript