v0.9.3: New widgets.Select; Clean-up widgets API.

[janpfeifer]

• Widgets:
  • Normalized API across widgets, so they all look the same -- breaks the API, apologies.
  • Added widget.Select for a drop-down menu.
  • Improved integration tests.
• nbexec: changed javascript code to use the public API jupyterapp.commands.execute instead.

---

This discussion was created from the release v0.9.3: New widgets.Select; Clean-up `widgets` API..