Upgrade to JupyterLab 2.0

[jtpio]

Listing the different steps to target JupyterLab 2.0:

• Update the @jupyterlab/... packages to -beta.2
• Switch to lumino: Switch to Lumino #208
• Services architecture rework (rename ClientSession to SessionContext): Rework kernel and session architecture jupyterlab#7252
• Remove Console specific toolbar logic (MainAreaWidget):

  debugger/src/index.ts

  Lines 83 to 89 in 793492b

  	const getToolbar = (): Toolbar => {
  	if (!(widget instanceof ConsolePanel)) {
  	return widget.toolbar;
  	}
  	const toolbar = widget.widgets.find(w => w instanceof Toolbar) as Toolbar;
  	return toolbar ?? new Toolbar();
  	};