Add a Property Inspector

[blink1073]

References

Fixes #7664

Code changes

Adds a property-inspector package and default implementation of the extension in application-extension. The notebook extension is updated to use the inspector.

/**
 * A property inspector interface provided when registering
 * to a property inspector provider.  Allows an owner widget
 * to set the property inspector content for itself.
 */
export interface IPropertyInspector extends IDisposable {
  /*
   * Render the property inspector content.
   *
   * If the owner widget is not the most recently focused,
   * The content will not be shown until that widget
   * is focused.
   *
   * @param content - the widget or react element to render.
   */
  render(content: Widget | React.ReactElement): void;

  /**
   * Show the property inspector panel.
   *
   * If the owner widget is not the most recently focused,
   * this is a no-op.  It should be triggered by a user
   * action.
   */
  showPanel(): void;
}

/**
 * A provider for property inspectors.
 */
export interface IPropertyInspectorProvider {
  /**
   * Register a widget in the property inspector provider.
   *
   * @param widget The owner widget whose properties will be inspected.
   *
   * ## Notes
   * Only one property inspector can be provided for each widget.
   * Registering the same widget twice will result in an error.
   * A widget can be unregistered by disposing of its property
   * inspector.
   */
  register(widget: Widget): IPropertyInspector;
}

User-facing changes

If no other extensions are installed that use the side panel, the use will always see the tool icon even when no notebooks are open, with the text "No properties to inspect.".

I attempted to only show the tab when one or more inspectors are registered, but it conflicts with our logic that allows side bar items to be moved to the opposite side.

Backwards-incompatible changes

None.

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[jasongrout]

The general functionality seems to work well. The notebook inspector needs some CSS/UX work:

And perhaps the wrench icon should be updated to something to more reflect a property inspector.

[jasongrout]

Approving for the API changes - we can take up the UX issues in another PR after the beta. This does need a rebase before merging, though.

[afshin]

Good to merge and iterate on after these changes.