Expand use of JupyterLab components

[bollwyvl]

References

• starts #46

Changes

• [x] use MainAreaWidget
  • [x] hoist that widget to manager
  • [x] name and hoist toolbar items from react to widgets
  • [x] host sidebar/main area toggle to separate widget
• [x] use ITranslator
  • [x] in main area widget
  • [x] in manager (e.g. error messages)
  • [x] in react components
• [x] only re-add widget to shell if area has changed